A Clam is a Hybrid Zone . It utilizes both man mark and zone defensive strategies.
 The Reasoning Behind a Clam
In Ultimate, as the thrower has up to ten seconds to make a pass, most players develop much more slowly. As the receivers remain open for a longer period of time, most throwing decisions can be well planned. The Clam attempts to make the Ultimate field look like a crowded football field. Receivers will break open, but only for a second, thus forcing the thrower to recognize the situation instantly as to whether to throw to the receiver or not.
Theoretically, the are always six potential receivers. At any given moment only one or two are viable threats. These threatening receivers have a limited number of specific cuts available. The Clam attempts to systematically shut down the most likely and most threatening cuts, leaving the low percentage and short yardage passes open. At the heart of the strategy, is a person to person defense, but which defensive player covers which offensive player isn’t determined until after the cuts have started.
Particular assignments are only valid as long as the offensive player is the most viable receiver in the defender’s area. It’s not unusual for one defender in the Clam to cover three different offensive positions during a ten second stall count. It’s also not unusual for a well-positioned defender to not cover anybody (and still be doing the job), since players often won’t cut into an area where there is already a defender.
Many variations of the Clam exist, but all require teamwork to succeed. Conventional person to person defense means there are seven offensive players matched against seven defenders. Most teams use either middle, sideline, forehand, or backhand force against the thrower. This is the first step toward reducing the area in which each defender has to cover.
With two players of comparable abilities, it is virtually impossible for the defense to prevent the offense from getting open. A cut is a foot race to a particular spot on the field, except that the offensive player doesn’t tell the defender which spot he or she is racing toward or when the race starts. The offensive player can change his or her mind anytime during the race.
Three people are in man-mark. They man mark the two people closest to the disc, and the player with possession. They generally mark handlers as they are the ones that are closest to the disc. The man-mark players also mark the dump, to prevent an easy swing to the break side.
The other four players are in zone defence. There is a deep, who stands deeper than the last offensive player. A break side wing, open side wing and a middle.
 Man Mark Mentality
The idea of the man-mark players is to cut down those immediate short options, to force the player with possession to look down field into the teeth of the zone. The player on the mark must be active not to let any easy break throw off, as that is the easiest way to beat the clam.
A force is called at the start of the point. That force must always be maintained and if a new player gets the disc, the man-mark players must get there quickly to get the force back on to prevent the offensive flow. The player on the mark should not bother tracking any hammer fakes, that is partly what you want the offense to do. A clam is normally played in wind, that combined with a half decent break side wing, any hammers should be hotly contested.
A trap could be used. However it is imperative that the break side is covered by the mark.
(Players 0,1 and 2 in the picture)
The middle (player 5 in the picture) covers the cuts up the middle of the field and prevents the defense from being busted up the middle. If the middle is perceptive and a good player, they will be able to hold their position whilst watching the disc. Then, if a risky pass is thrown into their area, they can cause the pass to be incomplete and get a turnover for the defense.
The middle also helps the open side wing cover cuts to the open side. Should a receiver cut short, (Open side wing covers) then cuts towards centre field, the middle should be aware and help out to allow the Open Side wing to return to his position and cover any more incoming cuts.
 Open Side Wing
The Open Side Wing (player 3 in the picture) covers all the cuts to the open side. They basically want to be already occupying the space the offensive recievers want to get into. They have the job of stopping any easy passes up field and basically clogging the whole open side offensive flow.
 Break Side Wing
The Break Side Wing (player 4) has a less active job. They cover the break side of the stack. The closer the disc is to the middle of the field, the more active the break side wing should be in tracking cuts. The further towards the open side sideline, the more the break side can just hang in the open side space. The middle should prevent any low break throws onto the break side. The break side wing covers anything up in the air.
One of the biggest killers in the clam is the cross field hammer. If the disc is bombed to a reciever from one side of the field to the other, the defence is a long way out of position and normally a point is scored. The break side wing must be aggressive in competing for anything that comes cross field (within the rules of the game). A clam is normally played in the wind, so any hammers thrown are normally majorly affected and are not that accurate.
The Break Side Wing should also be aware of anything deep on the break side behind the Deep. A cross field huck is rare, but there are players that can do it. Should it be thrown, the deep is rarely in a good position to get to the disc and the Break Side Wing is much better positioned.
The Deep (player 6) hangs slightly to the open side and covers any deep cuts. They play deeper than the last offensive man, and must cover the huck. The clam is normally beaten when the Open Side Wing or Deep are caught out of position. The Deep normally is one of the most gifted players, as they have to cover a lot of ground and must be able to win the jump disc. Often, the offense gets frustrated and tries to smash a pass through the zone either up the break side or deep. These hucks normally sit up in the wind, giving the deep time to make a play.
 Mixing it Up
As mentioned above, a Clam is most effective when played in the wind. It is also an effective tool for shutting down an offense that has been rolling straight over your defense. It changes the game to make the handlers throw more risky passes. It puts pressure on the entire offense. Not only to make a good pass, but to get free. A clam is rarely a zone to play for an entire game. A good offense will adjust to the Clam, given time. However, the handling mentality for a clam is different to the mentality used in a normal man on man defense. Used at opportune times, the Clam can completely stop the offensive flow of the other team and start to hand the momentum back to the defense.
It also serves to mix up the force. Step out with a backhand force when its been forehand all game and the offense throws the disc into the ground half the time. The other half the defense gets the turnover. It must be noted that the Deep must feel fit, because without question, if the offense gets half a chance, they will huck it deep.
Resources:  - The Ultimate HandBook: Advanced Defense:: The Clam.