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