A number of years pleasant, not one showing decline (9)
I believe the answer is:
'decline' is the definition.
(I've seen this before)
'a number of years pleasant not one' is the wordplay.
'a number of years' becomes 'decade' (I've seen this before).
'pleasant' becomes 'nice' (associated in meaning).
'not' is a deletion indicator.
'one' becomes 'I' (Roman numeral).
'nice' with 'i' removed is 'nce'.
'showing' is the link.
(Other definitions for decadence that I've seen before include "Perishing state" , "Debauchery, degeneracy" , "Degeneration" , "depravity" , "State of immorality and self-indulgence" .)