Cock's crest (4)
I believe the answer is:
comb
'cock's crest' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for comb that I've seen before include "Crest on the head of some birds" , "Used to tidy hair" , "Toothed grooming implement" , "Implement with teeth" , "Fastening spikes" .)