button up vs clam up

button up and clam up both are verbs.

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

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

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

button up (verb)	clam up (verb)
refuse to talk or stop talking; fall silent	refuse to talk or stop talking; fall silent