Excessive indulgence from the 80s perhaps, oddly niche (9)
I believe the answer is:
decadence
'excessive indulgence' is the definition.
The definition suggests a singular noun which matches the answer.
'80s perhaps oddly niche' is the wordplay.
'80s perhaps' becomes 'decade' (I can't justify this - if you can you should believe this answer much more).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternating letters of 'niche' are 'nce'.
'decade'+'nce'='DECADENCE'
'from the' is the link.
Can you help me to learn more?
(Other definitions for decadence that I've seen before include "depravity" , "Decline in quality" , "Immoral behaviour" , "Debauchery, degeneracy" , "Moral or cultural decline" .)