Old boy perceived, we hear, to be indecent (7)
I believe the answer is:
obscene
'be indecent' is the definition.
('obscene' can be similar in meaning to 'indecent')
'old boy perceived we hear' is the wordplay.
'old boy' becomes 'ob'.
'perceived' becomes 'seen' (I've seen this before).
'we hear' shows a homophone (sound like).
'seen' is a homophone of 'scene'.
'ob'+'scene'='OBSCENE'
'to' acts as a link.
(Other definitions for obscene that I've seen before include "Pseudonym" , "Lewd or loathsome" , "Lewd - offensive" , "Disgusting, repugnant" , "Sexually offensive" .)