Partitioned gym's set up over gallery (7)

'partitioned' is the definition.
(I've seen this in another clue)

'gym's set up over gallery' is the wordplay.
'gym's' becomes 'pe's' (short for Physical Education).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'pes' in reverse letter order is 'sep'.
'sep'+'tate'='SEPTATE'