Strong, lightweight wood (5)
I believe the answer is:
balsa
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strong lightweight wood</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for balsa that I've seen before include "Central American forest tree used to make floats, rafts, models etc" , "Strong, light wood" , "Tree yielding light wood" , "Lightweight timber" , "South American tree" .)