Fighting to finish in a knockout (5)
I believe the answer is:
kendo
'fighting' is the definition.
(I have seen 'Asian fighting' mean 'kendo' so perhaps 'fighting' could also mean 'kendo')
'finish in a knockout' is the wordplay.
'finish' becomes 'end' (synonyms).
'in' indicates putting letters inside.
'a knockout' becomes 'KO' (abbreviation).
'end' placed inside 'ko' is 'KENDO'.
'to' acts as a link.
(Other definitions for kendo that I've seen before include "wooden fencing?" , "Marrtial art" , "Fighting in Japan" , "Japanese fencing with bamboo staves" , "Japanese martial art using split bamboo swords" .)