Rafter; box (4)
I believe the answer is:
spar
This clue is a double definition.
'rafter' is the first definition.
(I've seen this before)
'box' is the second definition.
(spar can mean to box)
(Other definitions for spar that I've seen before include "Round pole to support sail" , "bar" , "Box; various minerals" , "Rafter, boom" , "Stout pole to support rigging" .)