Board of Trade (4)
I believe the answer is:
I believe this is a double definition.
'board' is the first definition.
(deal is a kind of board)
'trade' is the second definition.
('deal' can be a synonym of 'trade')
'of' acts as a link.
(Other definitions for deal that I've seen before include "Distribute playing cards" , "Kent resort" , "split deck" , "Soft wood; trade" , "A port, a bargain" .)