Decisive blow (4)
I believe the answer is:
kayo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">decisive blow</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a KO, a knock-out)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for kayo that I've seen before include "Stun" , "To render senseless" , "Knockout" , "Fight's ending" , "floor" .)