Rafter; box (4)
I believe the answer is:
spar
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">rafter</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">box</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(spar can mean to box)</span><br/ ><br/ ></span>
(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" .)