Complex old boy overlooking meeting (9)
I believe the answer is:
obsession
'complex' is the definition.
(I've seen this before)
'old boy overlooking meeting' is the wordplay.
'old boy' becomes 'ob'.
'overlooking' means one lot of letters go next to another (down clues only).
'meeting' becomes 'session' (session is a kind of meeting**).
'ob'+'session'='OBSESSION'
(Other definitions for obsession that I've seen before include "All-absorbing idea" , "Complex" , "Enthusiasm" , "Ruling passion" , "Fan may display this" .)