Colourful carp (3)
I believe the answer is:
koi
'colourful carp' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for koi that I've seen before include "Sort of carp" , "swimmer" , "Colourful ornamental carp" , "Carp; sounds like shy" , "Japanese type of carp" .)