Seat right next to organ (4)
I believe the answer is:
rear
'seat' is the definition.
('rear' can be a synonym of 'seat')
'right next to organ' is the wordplay.
'right next' becomes 'r' (I've seen this in other clues).
'to organ' becomes 'ear' (ear is a kind of organ**).
'r'+'ear'='REAR'
(Other definitions for rear that I've seen before include "Buck" , "what can be brought up, finally" , "Rump" , "Bring up (children)" , "Foster" .)