Trivial complaint (4)
(Other definitions for moan that I've seen before include "Bewail" , "Complaint" , "Complain; mournful sound" , "Complain; rhymes with money lent" , "Express pain" .)
'trivial 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 "Foolish person" , "Egg; fool" , "Cause of much head-scratching" , "small egg" , "Louse's egg; idiot" .)