clam upvi phrasalphrasal verb, intransitive: Verb with adverb(s) or preposition(s), having special meaning and not taking direct object--for example, "make up" [=reconcile]: "After they fought, they made up."
slang (be unable to speak)
（被询问时）缄口不言 bèi xún wèn shí jiān kǒu bù yán
拒不开口 jù bù kāi kǒu
Once he realizes everyone is listening to him, he will clam up.