Head-to-head combat (4)
I believe the answer is:
duel
'head-to-head combat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for duel that I've seen before include "Two-person combat" , "Contest between two people, perhaps with weapons" , "Fight to the death between two people" , "Affaire d'honneur" , "Contest to settle point of honour" .)