Resentment finally fading many years after failure (7)
I believe the answer is:
dudgeon
'resentment' is the definition.
(I've seen this before)
'finally fading many years after failure' is the wordplay.
'finally' says to take the final letters.
'many years' becomes 'eon' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'failure' becomes 'dud' (dud is a kind of failure).
The final letter of 'fading' is 'g'.
'g'+'eon'='geon'
'geon' put after 'dud' is 'DUDGEON'.
(Other definitions for dudgeon that I've seen before include "huff" , "(High) resentment" , "Deep resentment" , "State of being (highly) aggrieved" , "Offended indignation" .)