Nit-pick (5)
I believe the answer is:
cavil
I believe this is a double definition.
'nit' is the first definition.
(thesaurus)
'pick' is the second definition.
(I've seen this in another clue)
(Other definitions for cavil that I've seen before include "Raise petty objections" , "Minor complaint" , "Make trivial objection" , "Quibble" , "Make petty objections" .)