Meeting with former pupil first thing (9)
I believe the answer is:
obsession
'thing' is the definition.
(I've seen this before)
'meeting with former pupil' is the wordplay.
'meeting' becomes 'session' (session is a kind of meeting).
'with' is a charade indicator (letters next to each other).
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'session' put after 'ob' is 'OBSESSION'.
'first' is the link.
(Other definitions for obsession that I've seen before include "Phobia" , "Extreme interest" , "Mania" , "Enthusiasm" , "Ruling passion" .)