Indecent picture introduced by old boy (7)
I believe the answer is:
obscene
'indecent' is the definition.
('obscene' can be similar in meaning to 'indecent')
'picture introduced by old boy' is the wordplay.
'picture' becomes 'scene' (scene is a kind of picture).
'introduced by' says to put letters next to each other.
'old boy' becomes 'ob'.
'scene' after 'ob' is 'OBSCENE'.
(Other definitions for obscene that I've seen before include "Pseudonym" , "Sexually offensive" , "Improper" , "offending sensibilities?" , "Disgusting, repugnant" .)