Returning to fight off outcast (5)
I believe the answer is:
leper
'outcast' is the definition.
(leper is a kind of outcast)
'returning to fight off' is the wordplay.
'returning' says the letters should be written in reverse.
'to fight off' becomes 'repel' ('repel' can be a synonym of 'fight off').
'repel' in reverse letter order is 'LEPER'.
(Other definitions for leper that I've seen before include "Social reject" , "sick person" , "Undesirable" , "Outcast, pariah" , "Someone shunned" .)