Hock for man on board (4)
I believe the answer is:
pawn
This clue is a double definition.
'hock' is the first definition.
('pawn' can be a synonym of 'hock')
'man on board' is the second definition.
(chess)
'for' acts as a link.
(Other definitions for pawn that I've seen before include "Deposit as security for a loan" , "Give as security" , "Hock; cat's-paw" , "Deposit as pledge" , "a future queen, possibly" .)