Declining ten years with National Trust (8)
I believe the answer is:
decadent
'declining' is the definition.
(thesaurus)
'ten years with national trust' is the wordplay.
'ten years' becomes 'decade' (I've seen this before).
'with' means one lot of letters go next to another.
'national trust' becomes 'nt'.
'decade'+'nt'='DECADENT'
(Other definitions for decadent that I've seen before include "Abandoning principles" , "Morally corrupt" , "abandoned" , "Decanted in corrupt way" , "going off" .)