shut up vs clam up

shut up is an adjective but clam up is not an adjective.

Word	Noun	Adjective	Verb	Adverb
shut up	No	Yes	Yes	No
clam up	No	No	Yes	No

As verbs, shut up and clam up are synonyms defined as:

Other synonyms of shut up include be quiet, belt up, button up, close up, dummy up, keep mum.

shut up (adjective)	clam up (adjective)
closely confined

shut up (verb)	clam up (verb)
cause to be quiet or not talk	refuse to talk or stop talking; fall silent
refuse to talk or stop talking; fall silent
place in a place where something cannot be removed or someone cannot escape