Mounting or going down (7)
I believe the answer is:
setting
I believe this clue is a double definition.
'mounting' is the first definition.
(I know that setting is a type of mounting)
'going down' is the second definition.
(setting is a kind of going down)
'or' is the link.
(Other definitions for setting that I've seen before include "An event's location; hardening" , "approaching the horizon" , "Compiler's job" , "Background" , "musical version of poem" .)