Yard or boom, say (4)
I believe the answer is:
spar
I believe this is a double definition.
'yard' is the first definition.
(I've seen this in another clue)
'boom say' is the second definition.
(I know that boom is a type of spar)
(Other definitions for spar that I've seen before include "Box; mineral; pole" , "bar" , "Shadow-box" , "Scrap" , "Mast - boom - gaff" .)