Refuse to speak (4,2)
I believe the answer is:
clam up
'refuse to speak' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clam up that I've seen before include "'Fall silent and refuse to talk (4,2)'" , "Stop giving out" , "Refuse to talk like large shellfish?" , "Refuse to speak" , "Go silent" .)