Fast faller sung of at Cup Final (8)
I believe the answer is:
eventide
'fast' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand how the remainder of the clue works.
(Other definitions for eventide that I've seen before include "End of the day to the poet" , "The day's close (poetic)" , "onset of dusk" , "some time later?" , "when it starts to get dark" .)