Degenerate's time inside never to be forgotten (8)
I believe the answer is:
immortal
'never to be forgotten' is the definition.
(I've seen this before)
'degenerate's time' is the wordplay.
I cannot quite understand how this works, but
'time' could be 't' (abbreviation) and 't' is present in the answer.
'degenerate' could be 'immoral' (similar in meaning) and 'immoral' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'inside' acts as a link.
Can you help me to learn more?
(Other definitions for immortal that I've seen before include "Never dying" , "God" , "Undying" , "Imperishable" , "Living forever" .)