After ten years, a collection of books is deteriorating (8)
I believe the answer is:
decadent
'is deteriorating' is the definition.
(thesaurus)
'after ten years a collection of books' is the wordplay.
'after' is a charade indicator (letters next to each other).
'ten years' becomes 'decade' (a decade is ten years).
'a collection of books' becomes 'NT' (New Testament).
'decade' put next to 'nt' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "Marked by excessive self-indulgence and moral decay" , "Reflecting a state of moral decline" , "Luxuriously self-indulgent" , "In moral or cultural decline" , "Showing moral decline" .)