After ten years National Trust in state of decline (8)
I believe the answer is:
decadent
'in state of decline' is the definition.
(decadence can mean moral decline)
'after ten years national trust' is the wordplay.
'after' means one lot of letters go next to another.
'ten years' becomes 'decade' (a decade is ten years).
'national trust' becomes 'nt'.
'decade' put next to 'nt' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "Lacking in moral vigour" , "Debauched" , "Sybaritic" , "Decanted in corrupt way" , "Reflecting cultural decline" .)