Wood in fire is offered up (5)
I believe the answer is:
osier
'wood' is the definition.
(I've seen this before)
'in fire is offered up' is the wordplay.
'in' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'reiso' is hidden in the letters of 'fire is offered'.
'reiso' reversed gives 'OSIER'.
(Other definitions for osier that I've seen before include "Basket-making material" , "Tree - Rosie (anag)" , "Willow for basket-making" , "Basketry willow" , "Bit of tree" .)