Dirty old boy identified, you say? (7)
I believe the answer is:
obscene
'dirty' is the definition.
(similar in meaning)
'old boy identified you say?' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'identified' becomes 'seen' (identify can mean to recognise or see).
'you say?' shows a homophone (sound like).
'seen' sounds like 'scene'.
'ob'+'scene'='OBSCENE'
(Other definitions for obscene that I've seen before include "Shameless" , "Offensively vulgar" , "Lewd or loathsome" , "Pseudonym" , "Abhorrent or lewd" .)