keep mum vs clam up

keep mum and clam up both are verbs.

Word	Noun	Adjective	Verb	Adverb
keep mum	No	No	Yes	No
clam up	No	No	Yes	No

As verbs, keep mum and clam up are synonyms defined as:

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

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