Dark autumn evening (9)
I believe the answer is:
nightfall
'evening' is the definition.
(I've seen this before)
'dark autumn' is the wordplay.
'dark' becomes 'night' (it's dark at nighttime).
'autumn' becomes 'fall' (synonyms).
'night'+'fall'='NIGHTFALL'
(Other definitions for nightfall that I've seen before include "gloaming" , "Dusk" , "when the light fails" , "Evening" , "start of the dark time" .)