Rising start of social outcast sickens (6)
I believe the answer is:
repels
'sickens' is the definition.
(sicken can mean to repel or disgust)
'rising start of social outcast' is the wordplay.
'rising' says the letters should be written backwards (going up in a down clue).
'start of' says to take the initial letters.
'outcast' becomes 'leper' (leper is a kind of outcast).
The first letter of 'social' is 's'.
's'+'leper'='sleper'
'sleper' written backwards gives 'REPELS'.
(Other definitions for repels that I've seen before include "Drives away from lepers" , "Drives back or disgusts (6)" , "Forces back" , "Disgusts or drives away" .)