Declining environment? (7)
I believe the answer is:
setting
I believe this is a double definition.
'declining' is the first definition.
(thesaurus)
'environment?' is the second definition.
(I know that setting is a type of environment)
(Other definitions for setting that I've seen before include "Gem''s mounting - stage scenery" , "Background" , "Mount for gem" , "Dropping beneath the horizon" , "musical version of poem" .)