One-on-one combat (4)
I believe the answer is:
duel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one-on-one combat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for duel that I've seen before include "Combat to settle a row" , "Contest between two people, perhaps with weapons" , "Potentially fatal fight between two people" , "shoot-out?" , "Affair of honour (between two men)" .)