What does 'Clam down' mean?

"Clam down" would not be a proper English phrase although "clam up" is, and it means to quickly become quiet. If you meant "calm down" and you misspelled then it means to relax and stop worrying. Example "calm down you only misspelled one word, I think your doing a great job!"

It means to not be loud and to relax

It means to not be loud and to relax

This is phrasal verb. It means: to be, or to become calm and stop worrying.