Bug old boy before meeting (9)
I believe the answer is:
obsession
'bug' is the definition.
(someone with a bug for something has an obsession)
'old boy before meeting' is the wordplay.
'old boy' becomes 'ob'.
'before' says to put letters next to each other.
'meeting' becomes 'session' (session is a kind of meeting**).
'ob'+'session'='OBSESSION'
(Other definitions for obsession that I've seen before include "Mania" , "Fan may display this" , "Continual preoccupation" , "Phobia" , "Fixed idea" .)