Phobia of old boy going to meeting (9)
I believe the answer is:
obsession
'phobia' is the definition.
(I've seen this before)
'old boy going to meeting' is the wordplay.
'old boy' becomes 'ob'.
'going to' means one lot of letters go next to another.
'meeting' becomes 'session' (session is a kind of meeting**).
'ob'+'session'='OBSESSION'
'of' acts as a link.
(Other definitions for obsession that I've seen before include "Extreme interest" , "Enthusiasm" , "Ruling passion" , "Thing" , "Persistent preoccupation" .)