Nag a swimmer (4)

I believe the answer is:

carp

I believe this is a double definition.

'nag' is the first definition.
(I've seen this in another clue)

'a swimmer' is the second definition.
(carp is a kind of fish)

(Other definitions for carp that I've seen before include "Go on at" , "Find fault with a freshwater fish" , "ish" , "Fish; find fault continually" , "Fish - grouse" .)

I've seen this clue in The Everyman puzzle in The Observer.

Want a hint initially instead of a full solution? Install my app