Horseman on board? (6)
I believe the answer is:
knight
This might be a double definition.
'horseman' is the first definition.
(knight on horseback)
'board?' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'on' is the link.
(Other definitions for knight that I've seen before include "Holder of non-hereditary title in UK" , "In Britain, a person honoured by the sovereign for personal merit" , "'Chessman, the horse's head (6)'" , "Male honoured by the British sovereign" , "Rank below baronet" .)