Dark period man on board talked of (5)
I believe the answer is:
night
'dark period' is the definition.
(period of the day when it's dark)
'man on board talked of' is the wordplay.
'man on board' becomes 'knight' (chess piece - man can mean a chess piece).
'talked of' shows a homophone (sound like).
'knight' is a homophone of 'NIGHT'.
(Other definitions for night that I've seen before include "Darkness; see you in the morning" , "A Hard Day's (the Beatles)" , "See 8" , "Time for bed" , "'The 2 8 of the dog in the _____'" .)