Process to extract metal gave off noxious fumes (5)
I believe the answer is:
smelt
I believe this clue is a double definition.
'process to extract metal' is the first definition.
'gave off noxious fumes' is the second definition.
(smelt, was smelly)
(Other definitions for smelt that I've seen before include "Produce metal by heat" , "18 [EXTRACT] metal" , "23 [FISH]" , "Inhaled odour" , "Small fish of salmon species" .)