A thick piece, turned wood (5)
I believe the answer is:
balsa
'wood' is the definition.
(type of wood)
'a thick piece turned' is the wordplay.
'thick piece' becomes 'slab' (I've seen this in another clue).
'turned' shows that the letters should be reversed in order.
'a'+'slab'='aslab'
'aslab' reversed gives 'BALSA'.
(Other definitions for balsa that I've seen before include "Very lightweight wood from tropical tree" , "Tree yielding light wood" , "Wood used in model making" , "South American tree" , "Raft-making S American corkwood" .)