Keep man on board (6)
I believe the answer is:
castle
This clue is a double definition.
'keep' is the first definition.
(I've seen this before)
'man on board' is the second definition.
(man can mean a chess piece)
(Other definitions for castle that I've seen before include "a piece in the corner" , "Move king to safety - stronghold" , "Stronghold - chessman" , "Man on board" , "Large fortified building" .)