Carp in a pond (3)
I believe the answer is:
koi
'carp in a pond' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for koi that I've seen before include "Colourful ornamental carp" , "Ornamental carp variety" , "swimmer" , "Kind of decorative carp" , "Sort of carp" .)