Hello, Could you provide us with some context? Where did you hear this phrase and what was the full sentence? There are quite a few (American?) slang terms which use the word "clam" such as "clam up" and "clam it", but I've only ever hear them in films. They all carry the same meaning which is "Shut up and calm down!" "Clam down" could possibly be a variation? http://reefcorner.com/wp-content/uploads/Crocea-clam-closed-512x384.jpg When someone says this, the person the phrase is directed at should act like a clam and keep their mouths shut tight! ;)

I'm guessing you mean "clam up," as Michael Wheeler says. It's a familiar U.S. colloquial expression, but, also as Wheeler says, I'm actually sure I've heard it spoken in real life. To me it carries a sense of "to stop talking BECAUSE you are about to reveal a secret." Google News finds me some examples: "doctors treating illegals are being told to clam up" (keep it secret) "Politicians must choose words carefully to protect society... Perhaps the best way is to just clam up and avoid gaffes altogether." (A "gaffe" is an embarrassing mistake). "How to talk about money before you marry... Your future mate will most likely clam up the second they feel they are being interrogated."

I've never heard of 'clam down'. Do you mean 'clamp down'?

Maybe you meant "calm down"? If that's the case, it means "to become less angry/emotional/irritated" Example: "I know your wife has just left you but you really have to calm down."