Light wood (5)
I believe the answer is:
balsa
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">light wood</span>' is the definition.<br/ ><span class="explanation-format__explanation">(balsa is a light wood used in construction)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for balsa that I've seen before include "Wood used in model making" , "Tree yielding light wood" , "Very light timber" , "South American tree" , "Strong lightweight wood" .)