belt up vs clam up

belt up and clam up both are verbs.

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

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

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

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