Expected to end in mortal combat (4)
I believe the answer is:
duel
'combat' is the definition.
(duel is a kind of combat)
'expected to end in mortal' is the wordplay.
'expected' becomes 'due' (I've seen this before).
'to end in' says to take the final letters.
The last letter of 'mortal' is 'l'.
'due'+'l'='DUEL'
(Other definitions for duel that I've seen before include "Two-person combat" , "Combat between two people in matter of honour" , "One got satisfaction from this" , "Fight between two persons" , "shoot-out?" .)