Indecent old boy sighted, we hear (7)
I believe the answer is:
obscene
'indecent' is the definition.
('obscene' can be similar in meaning to 'indecent')
'old boy sighted we hear' is the wordplay.
'old boy' becomes 'ob'.
'sighted' becomes 'seen' (I've seen this before).
'we hear' shows a homophone (sound like).
'seen' is a homophone of 'scene'.
'ob'+'scene'='OBSCENE'
(Other definitions for obscene that I've seen before include "Prurient" , "Offensive - indecent" , "Shameless" , "Dirty" , "Rude" .)