Witches head off to furnace (4)
I believe the answer is:
oven
'to furnace' is the definition.
(I've seen this before)
'witches head off' is the wordplay.
'witches' becomes 'coven' (I've seen this before).
'head off' means to remove the first letter.
'coven' with its initial letter removed is 'OVEN'.
(Other definitions for oven that I've seen before include "Small kiln" , "Kiln, cooker" , "kitchen item" , "Compartment for baking" , "Cooking appliance" .)