Former pupil meeting shows passion (9)
I believe the answer is:
obsession
'passion' is the definition.
(thesaurus)
'former pupil meeting' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'meeting' becomes 'session' (session is a kind of meeting).
'ob'+'session'='OBSESSION'
'shows' is the link.
(Other definitions for obsession that I've seen before include "Fan may display this" , "Ruling passion" , "Extreme interest" , "All-absorbing idea" , "Thing" .)