Salacious old boy, part of act on stage (7)
I believe the answer is:
obscene
'salacious' is the definition.
(synonyms)
'old boy part of act on stage' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'part of act on stage' becomes 'scene' (I can't explain this - if you can you should give a lot more credence to this answer).
'ob'+'scene'='OBSCENE'
Can you help me to learn more?
(Other definitions for obscene that I've seen before include "Shocking" , "X-rated" , "One's CBE (anag)" , "offending sensibilities?" , "Morally offensive, disgusting" .)