Stop talking (4,2)
'stop talking' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clam up that I've seen before include "Refuse to talk like large shellfish?" , "Abruptly stop talking (colloq.)" , "Stop giving out" , "'Fall silent and refuse to talk (4,2)'" , "Keep mum" .)
'stop talking' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pipe down that I've seen before include "make less noise" , "Stop talking" , "Shut up!" , "Be quiet!" , "Say no more!" .)