clam up vs shut up

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

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

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

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

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

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