Fixed idea of old boy ahead of meeting (9)
I believe the answer is:
obsession
'fixed idea' is the definition.
(I've seen this before)
'old boy ahead of meeting' is the wordplay.
'old boy' becomes 'ob'.
'ahead of' is a charade indicator (letters next to each other) (I've seen this in other clues).
'meeting' becomes 'session' (session is a kind of meeting).
'ob'+'session'='OBSESSION'
'of' is the link.
(Other definitions for obsession that I've seen before include "Fan may display this" , "All-absorbing idea" , "Complex" , "Continual preoccupation" , "Phobia" .)