Darkest time man on board reported (5)
I believe the answer is:
night
'darkest time' is the definition.
'night' can be an answer for 'time' (night is a kind of time). I'm unsure of the 'darkest' bit.
'man on board reported' is the wordplay.
'man on board' becomes 'knight' (chess piece - man can mean a chess piece).
'reported' indicates a 'sounds like' (homophone) clue.
'knight' sounds like 'NIGHT'.
Can you help me to learn more?
(Other definitions for night that I've seen before include "End of the day" , "'She walks in beauty, like the . . . . .' (Byron)" , "Period of darkness" , "Hours of darkness" , "Time without 10" .)