Fish – find fault (4)
I believe the answer is:
carp
I believe this clue is a double definition.
'fish' is the first definition.
(I know that carp is a type of fish)
'find fault' is the second definition.
(I've seen this before)
(Other definitions for carp that I've seen before include "Continually nag; fish" , "Go on at" , "Keep complaining" , "Moan; fish" , "Fish - make captious criticism" .)