School officer on ecstasy (7)
I believe the answer is:
college
'school' is the definition.
(college is a kind of school)
'officer on ecstasy' is the wordplay.
'officer' becomes 'col' (short for colonel).
'on' becomes 'leg' (cricket term, fielding positions).
'ecstasy' becomes 'E' (informal term for the drug).
'col'+'leg'+'e'='COLLEGE'
(Other definitions for college that I've seen before include "Where to train" , "Education institute" , "Balliol, perhaps?" , "Place of learning" , "pudding?" .)