Use last men up on unknown opponent (5)
I believe the answer is:
enemy
'opponent' is the definition.
(I know that enemy is a type of competitor)
'use last men up on unknown' is the wordplay.
'last' suggests the final letters.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
The last letter of 'use' is 'e'.
'men' written backwards gives 'nem'.
'e'+'nem'+'y'='ENEMY'
(Other definitions for enemy that I've seen before include "Armed adversary" , "Time" , "Division" , "Hostile person, foe" , "Antagonist" .)