Disgust of outcast getting knocked back (5)
I believe the answer is:
repel
'disgust' is the definition.
('repel' can be a synonym of 'disgust')
'outcast getting knocked back' is the wordplay.
'outcast' becomes 'leper' (leper is a kind of outcast).
'getting knocked back' says the letters should be written backwards.
'leper' backwards is 'REPEL'.
'of' acts as a link.
(Other definitions for repel that I've seen before include "Drive away like leper" , "Reject" , "Be effective in controlling" , "Drive or force back" , "Sicken" .)