clam up vs keep mum

clam up and keep mum both are verbs.

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

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

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

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