Driving away fast, one avoided reversing first (9)
I believe the answer is:
repellent
'driving away' is the definition.
(that which repels)
'fast one avoided reversing first' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'one avoided' becomes 'leper' (leper can figuratively mean a person people avoid).
'reversing' is a reversal indicator.
'first' means one lot of letters go next to another.
'leper' backwards is 'repel'.
'lent' put after 'repel' is 'REPELLENT'.
(Other definitions for repellent that I've seen before include "Obnoxious" , "It arouses distaste or drives insects away" , "Hideous" , "Disgusting, or substance that drives away insects" , "Repugnant, loathsome" .)