The dark chessman it's said (5)
I believe the answer is:
night
'the dark' is the definition.
(it's dark at nighttime)
'chessman it's said' is the wordplay.
'chessman' becomes 'knight' (knight is a kind of chessman).
'it's said' indicates a 'sounds like' (homophone) clue.
'knight' is a homophone of 'NIGHT'.
(Other definitions for night that I've seen before include "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "''Tender is the ...'' (F Scott Fitzgerald)|" , "follow-up to 16 [TWILIGHT]" , "The dark time" , "late" .)