Chick sound (4)
I believe the answer is:
peep
'chick sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for peep that I've seen before include "Look furtively or voyeuristically" , "Look surreptitiously, like Tom" , "Secret look" , "Look secretly" , "Large underground chamber" .)