Tiny complaint (3)
I believe the answer is:
nit
'tiny complaint' 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" , "Louse's egg; idiot" , "Parasite's egg made of tin" , "Louse's egg - berk" , "Muggins" .)