One-on-one fight (4)
I believe the answer is:
duel
'one-on-one fight' 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" , "Single combat" , "Personal combat" , "Contest between two people, perhaps with weapons" , "Combat between two people in matter of honour" .)