Declining ten years with National Trust (8)
I believe the answer is:
Here is my best explanation:
'declining' is the definition.
'ten years with national trust' is the subsidiary indication.
'ten years' becomes 'decade' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'national trust' becomes 'nt'.
(Other definitions for decadent that I've seen before include "Luxuriously self-indulgent", "Declining", "Morally deficient; declining", "going off", "Symbolist", "Immoral", "Reflecting cultural decline", "Corrupt", "lacking in moral vigour" and "Decanted, being corrupt and immoral".)