Light wood, a thick piece turned over (5)
I believe the answer is:
balsa
'light wood' is the definition.
(I've seen this before)
'a thick piece turned over' is the wordplay.
'thick piece' becomes 'slab'.
'turned over' says the letters should be written in reverse.
'a'+'slab'='aslab'
'aslab' written backwards gives 'BALSA'.
(Other definitions for balsa that I've seen before include "Lightweight timber" , "Strong lightweight wood" , "Wood used in model making" , "Very light timber" , "Something light" .)