Man on board heading off in the dark (5)
I believe the answer is:
night
'the 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 first letter removed is 'NIGHT'.
'in' acts as a link.
(Other definitions for night that I've seen before include "The dark time" , "A Hard Day's (the Beatles)" , "That's when it's darkest" , "'She walks in beauty, like the . . . . .' (Byron)" , "Evening" .)