Sets up orchestra partly for backing musical work (4)
I believe the answer is:
opus
'musical work' is the definition.
(synonyms)
'sets up orchestra partly for backing' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'for backing' says the letters should be written in reverse.
'supo' is hidden in the letters of 'sets up orchestra'.
'supo' written backwards gives 'OPUS'.
(Other definitions for opus that I've seen before include "A Latin work" , "Musical piece" , "Composer's work" , "Piece of music (with number)" , "Musical composition in the soup" .)