Dark man on board heading off (5)
I believe the answer is:
night
'dark' is the definition.
(night is a kind of dark)
'man on board heading off' is the wordplay.
'man on board' becomes 'knight' (I've seen this before).
'heading off' means to remove the first letter.
'knight' with its initial letter taken away is 'NIGHT'.
(Other definitions for night that I've seen before include "Time without 10" , "Time between days" , "Day's partner" , "Dark hours" , "period of gloom" .)