Furnace, essentially (4)
I believe the answer is:
oven
'furnace essentially' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oven that I've seen before include "Kiln, cooker" , "Cooker; furnace" , "Part of a cooker" , "kitchen item" , "Metal box in which to roast" .)