Catch at small faults in fish (4)
I believe the answer is:
carp
This clue could be a double definition.
'catch at small' is the first definition.
'carp' can be an answer for 'catch' (I've seen this before). I'm not sure about the 'at small' bit.
'faults in fish' is the second definition.
'carp' can be an answer for 'fish' (carp is a kind of fish). I'm not certain of the 'faults in' bit.
Can you help me to learn more?
(Other definitions for carp that I've seen before include "Fish - make captious criticism" , "Criticise - fish" , "praise? The opposite" , "Fish; nag pettily" , "Fish - grouse" .)