Decline, deteriorate (6)
I believe the answer is:
worsen
This clue is a double definition.
'decline' is the first definition.
(I know that decline can be written as worsen)
'deteriorate' is the second definition.
(I know that deteriorate is a more specific form of the action the worsen)
(Other definitions for worsen that I've seen before include "Deteriorate for owners" , "Become more inferior" , "go downhill" , "Go to pot" , "Become less well" .)