Japanese fighting finish in knockout (5)
I believe the answer is:
kendo
'japanese fighting' is the definition.
(kendo is a martial art)
'finish in knockout' is the wordplay.
'finish' becomes 'end' (synonyms).
'in' means one lot of letters goes inside another.
'knockout' becomes 'KO' (abbreviation).
'end' inserted inside 'ko' is 'KENDO'.
(Other definitions for kendo that I've seen before include "Martial art involving stick fighting" , "Fighting in Japan" , "Marrtial art" , "Asian sport" , "Japanese fencing with bamboo staves" .)