Fixed idea of old boy prior to meeting (9)
I believe the answer is:
obsession
'fixed idea' is the definition.
(I've seen this before)
'old boy prior to meeting' is the wordplay.
'old boy' becomes 'ob'.
'prior to' says to put letters next to each other.
'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 "Compulsion, fixation" , "Thing" , "Mania" , "Extreme interest" , "Phobia" .)