Fight outstanding learner (4)
I believe the answer is:
duel
'fight' is the definition.
(I know that duel is a type of fight)
'outstanding learner' is the wordplay.
'outstanding' becomes 'due' (an outstanding debt is due).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'due'+'l'='DUEL'
(Other definitions for duel that I've seen before include "Potentially fatal fight between two people" , "Contest between two with deadly weapons" , "Fight between two people over an affair of honour" , "Combat between two people in matter of honour" , "Arranged fight" .)