Dark period man on board reported (5)
I believe the answer is:
night
'dark period' is the definition.
(period of the day when it's dark)
'man on board reported' is the wordplay.
'man on board' becomes 'knight' (chess piece - man can mean a chess piece).
'reported' shows a homophone (sound like).
'knight' sounds like 'NIGHT'.
(Other definitions for night that I've seen before include "8d [DARKNESS]" , "Day's partner" , "Ignorance" , "- School, - owl" , "The dark time" .)