Play Othello, say, when it's mounted (4)
I believe the answer is:
room
'play' is the definition.
(Harold Pinter's first play, The Room)
'othello say when it's mounted' is the wordplay.
'othello say' becomes 'moor' (historical group of Muslims).
'when it's mounted' is a reversal indicator.
'moor' reversed gives 'ROOM'.
(Other definitions for room that I've seen before include "Enclosed indoor space" , "Box ---, storage area" , "kitchen?" , "Space; chamber" , "Adequate space" .)