Strong, lightweight wood (5)
I believe the answer is:
balsa
'strong lightweight wood' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)