Man decapitated before autumn, when it gets dark (9)
I believe the answer is:
nightfall
'when it gets dark' is the definition.
'man decapitated before autumn' is the wordplay.
'man' becomes 'knight' (man can mean a piece in chess).
'decapitated' means to remove the first letter.
'before autumn' becomes 'fall' ('fall' can be a synonym of 'autumn'. I am not sure about the 'before' bit.**).
'knight' with its initial letter removed is 'night'.
'night'+'fall'='NIGHTFALL'
(Other definitions for nightfall that I've seen before include "when visibility reduces" , "End of day" , "when the light fails" , "gloaming" , "start of the dark time" .)