Small grouse (3)
I believe the answer is:
nit
'small grouse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nit that I've seen before include "Idiot, ass" , "Muggins" , "Stupid person; egg" , "Cause of much head-scratching" , "Parasite's egg made of tin" .)