A solid block set up in wood (5)
I believe the answer is:
balsa
'wood' is the definition.
(type of wood)
'a solid block set up' is the wordplay.
'solid block' becomes 'slab' (eg a slab of stone).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'a'+'slab'='aslab'
'aslab' backwards is 'BALSA'.
'in' acts as a link.
(Other definitions for balsa that I've seen before include "Wood used by model makers" , "wood for raft" , "Something light" , "Very light timber" , "South American tree" .)