Find fault to a fault (4)
I believe the answer is:
carp
'find fault to a fault' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for carp that I've seen before include "Fish - make captious criticism" , "ish" , "Fish < and beef" , "Moan; fish" , "Raise trivial objections" .)