Rude old boy on site (7)
I believe the answer is:
obscene
'rude' is the definition.
(I've seen this before)
'old boy on site' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'on' means one lot of letters go next to another.
'site' becomes 'scene' (both can mean the location for something).
'ob'+'scene'='OBSCENE'
(Other definitions for obscene that I've seen before include "Lewd - offensive" , "Lewd or loathsome" , "Rude" , "Salacious" , "off colour?" .)