Old boy sitting on "sitting thing" (9)
I believe the answer is:
obsession
'thing' is the definition.
(I've seen this before)
'old boy sitting on sitting' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'sitting on' means one lot of letters go next to another.
'sitting' becomes 'session' (eg a parliamentary session).
'ob'+'session'='OBSESSION'
(Other definitions for obsession that I've seen before include "Mania" , "Thing" , "Unhealthy compulsion" , "Fan may display this" , "All-consuming idea" .)