Former pupil regarded, reportedly, as vulgar (7)
I believe the answer is:
obscene
'vulgar' is the definition.
(thesaurus)
'former pupil regarded reportedly' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'regarded' becomes 'seen' (I've seen this in another clue).
'reportedly' shows a homophone (sound like).
'seen' sounds like 'scene'.
'ob'+'scene'='OBSCENE'
'as' acts as a link.
(Other definitions for obscene that I've seen before include "Rude" , "unsuitable for children?" , "Salacious" , "Indecent or pornographic" , "offending sensibilities?" .)