Fight off rising outcast (5)
I believe the answer is:
repel
'fight off' is the definition.
(synonyms)
'rising outcast' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'outcast' becomes 'leper' (leper is a kind of outcast).
'leper' in reverse letter order is 'REPEL'.
(Other definitions for repel that I've seen before include "Fight off - drive back" , "Drive back like leper" , "Drive back or away" , "Drive back - revolt" , "give you the creeps" .)