One-on-one combat (4)
I believe the answer is:
duel
'one-on-one combat' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)