King, dark chessman (6)
I believe the answer is:
knight
'chessman' is the definition.
(I know that knight is a type of chessman)
'king dark' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'dark' becomes 'night' (it's dark at nighttime).
'k'+'night'='KNIGHT'
(Other definitions for knight that I've seen before include "in quest for a mate" , "Heroic champion" , "Chesspiece - person dubbed" , "soldier" , "Honour" .)