A piece of cake being raised in a wood (5)
I believe the answer is:
balsa
'a wood' is the definition.
(type of wood)
'a piece of cake being raised' is the wordplay.
'piece of cake' becomes 'slab' (large slice of cake).
'being raised' says the letters should be written in reverse (going up in a down clue).
'a'+'slab'='aslab'
'aslab' reversed gives 'BALSA'.
'in' acts as a link.
(Other definitions for balsa that I've seen before include "Very light wood in a slab" , "Very lightweight wood from tropical tree" , "Strong lightweight wood" , "Tropical American tree" , "Wood used by model makers" .)