He's backing music, say, by monster band (9)
I believe the answer is:
orchestra
'band' is the definition.
(musical band)
'he's backing music say by monster' is the wordplay.
'backing' shows that the letters should be reversed in order.
'music say' becomes 'art' (music is a kind of art).
'by' means one lot of letters go next to another.
'monster' becomes 'orc' (fantasy creature).
'art' backwards is 'tra'.
'hes'+'tra'='hestra'
'hestra' after 'orc' is 'ORCHESTRA'.
(Other definitions for orchestra that I've seen before include "Music ensemble" , "eg Halle" , "bandsmen?" , "in four sections" , "Eg, LSO, RPO" .)