Play instrument, backing … (4)
I believe the answer is:
loot
'play' is the definition.
(I've seen this before)
'instrument backing' is the wordplay.
'instrument' becomes 'tool' ('tool' can be a synonym of 'instrument').
'backing' is a reversal indicator.
'tool' backwards is 'LOOT'.
(Other definitions for loot that I've seen before include "Plunder, pillage" , "Tool for plundering" , "Prize" , "Steal (goods); money" , "Plunder - booty" .)