Raft wood (5)
I believe the answer is:
balsa
'raft wood' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for balsa that I've seen before include "Tree yielding light wood" , "lightweight boat-builder" , "Very light wood in a slab" , "Raft-making S American corkwood" , "Tropical American tree" .)