Cavil about the fish (4)
I believe the answer is:
carp
'cavil about the fish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for carp that I've seen before include "Cavil - fish" , "Complain in manner of goldfish" , "Pond fish" , "Fish often kept in ponds" , "Fish; find fault continually" .)