Gradual decline (7)
I believe the answer is:
erosion
'gradual decline' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erosion that I've seen before include "dying away" , "Wearing away (of soil, rocks etc)" , "Mechanical process of wearing something down" , "Abrasion" , "being eaten away" .)