Stun one carp (3)
I believe the answer is:
koi
'carp' is the definition.
(I've seen this before)
'stun one' is the wordplay.
'stun' becomes 'KO' (knock out in boxing).
'one' becomes 'i' (Roman numeral).
'ko'+'i'='KOI'
(Other definitions for koi that I've seen before include "Japanese fish" , "swimmer" , "Ornamental carp variety" , "Sort of carp" , "Kind of decorative carp" .)