Petty complaint (3)
I believe the answer is:
nit
'petty complaint' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nit that I've seen before include "Eg of louse" , "Louse; fool" , "Stupid person; egg" , "Parasite's egg made of tin" , "Muggins" .)