Repugnant prospect offered to old boy (7)
I believe the answer is:
obscene
'repugnant' is the definition.
('obscene' can be a synonym of 'repugnant')
'prospect offered to old boy' is the wordplay.
'prospect' becomes 'scene' ('scene' can be a synonym of 'prospect').
'offered to' is a charade indicator (letters next to each other).
'old boy' becomes 'ob'.
'scene' after 'ob' is 'OBSCENE'.
(Other definitions for obscene that I've seen before include "not decent" , "Bawdy" , "Morally offensive, disgusting" , "Lewd - offensive" , "blue" .)