Offensive former pupil's poisonous (9)
I believe the answer is:
obnoxious
'offensive' is the definition.
('obnoxious' can be similar in meaning to 'offensive')
'former pupil's poisonous' is the wordplay.
'former pupil' becomes 'ob' (abbreviation for 'Old Boy').
'poisonous' becomes 'noxious' (I've seen this before).
'ob'+'noxious'='OBNOXIOUS'
(Other definitions for obnoxious that I've seen before include "Foul" , "Seriously disliked" , "Loathsome" , "Repugnant" , "Nasty" .)