Fish - or nag? (4)
I believe the answer is:
carp
This clue is a double definition.
'fish' is the first definition.
(I know that carp is a type of fish)
'nag?' is the second definition.
(I've seen this in another clue)
'or' acts as a link.
(Other definitions for carp that I've seen before include "Find fault with a freshwater fish" , "Complain continually" , "Swimmer" , "Raise petty objections about goldfish" , "praise? The opposite" .)