Box; beam (4)
I believe the answer is:
spar
This clue is a double definition.
'box' is the first definition.
(I know that spar is a type of box)
'beam' is the second definition.
(I've seen this before)
(Other definitions for spar that I've seen before include "Mineral; strong pole" , "Pole to support sails, rigging etc" , "Train at boxing; mineral" , "Round pole to support sail" , "Raps with strong pole" .)