For a number of years, nut case is overly self-indulgent (8)
I believe the answer is:
decadent
'self-indulgent' is the definition.
(similar in meaning)
'for a number of years nut case is overly' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'number' could be 'e' (e is a kind of number) and 'e' is present in the answer.
'nut' could be 'en' ('nut' is printers' slang for an en space) and 'en' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for decadent that I've seen before include "Rotten" , "Self-indulgent or immoral" , "abandoned" , "Decanted in corrupt way" , "Dissolute and corrupt" .)