Reject outcast swapping sides (5)
I believe the answer is:
repel
'swapping sides' is the definition.
I can't tell whether this definition defines the answer.
'reject outcast' is the wordplay.
'reject' says the letters should be written in reverse.
'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 "Reject" , "Fill with distaste" , "Drive back or disgust" , "Fight off - drive back" , "Drive or force back" .)