Former pupil finds hearing complex (9)
I believe the answer is:
obsession
'complex' is the definition.
(I've seen this before)
'former pupil finds hearing' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'finds' is a charade indicator (letters next to each other).
'hearing' becomes 'session' (I've seen this before).
'ob'+'session'='OBSESSION'
(Other definitions for obsession that I've seen before include "Compulsion, fixation" , "Phobia" , "Thing" , "Mania" , "Unhealthy compulsion" .)