Pledge that may be taken on board (4)
I believe the answer is:
pawn
'pledge' is the definition.
(I know that pawn is a type of pledge)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pawn that I've seen before include "Deposit as security for a loan" , "piece on board" , "The foot-soldier in chess" , "Chessman often sacrificed" , "Chesspiece" .)