Conservationists after ten years in decline (8)
I believe the answer is:
decadent
'in decline' is the definition.
(I've seen this before)
'conservationists after ten years' is the wordplay.
'conservationists' becomes 'nt' (abbreviation for National Trust).
'after' says to put letters next to each other.
'ten years' becomes 'decade' (I've seen this before).
'nt' after 'decade' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "In moral or cultural decline" , "Self-indulgent or immoral" , "Decanted, being corrupt and immoral" , "Morally deficient; declining" , "base?" .)