Better to catch river fish (4)
I believe the answer is:
carp
'fish' is the definition.
(carp is a kind of fish)
'better to catch river' is the wordplay.
'better' becomes 'cap' (I've seen this before).
'to catch' indicates putting letters inside.
'river' becomes 'r'.
'cap' going around 'r' is 'CARP'.
(Other definitions for carp that I've seen before include "Raise petty objections about goldfish" , "praise? The opposite" , "Find fault with a freshwater fish" , "Pond fish" , "Keep complaining" .)