Martial art is to finish in decisive blow (5)
I believe the answer is:
kendo
'martial art' is the definition.
(I've seen this before)
'finish in decisive blow' is the wordplay.
'finish' becomes 'end' (synonyms).
'in' means one lot of letters goes inside another.
'decisive blow' becomes 'KO' (knock out blow in boxing).
'end' inserted into 'ko' is 'KENDO'.
'is to' is the link.
(Other definitions for kendo that I've seen before include "Combat" , "Japanese fighting" , "Japanese form of fencing" , "Asian fighting" , "Japanese martial art using split bamboo swords" .)