Declining music for a song (7)
I believe the answer is:
setting
'declining' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for setting that I've seen before include "Mount for gem" , "Where one may find Ruby" , "Musical version of song" , "Environment" , "Writing puzzles, perhaps" .)