Disgusting old boy on location (7)
I believe the answer is:
obscene
'disgusting' is the definition.
(obscene can mean disgusting or indecent)
'old boy on location' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'location' becomes 'scene' (scene is a kind of location).
'ob'+'scene'='OBSCENE'
(Other definitions for obscene that I've seen before include "Rude" , "Offensive - indecent" , "X-rated" , "not decent" , "Shocking" .)