Smelly and hard, giving off sulphur (4)
I believe the answer is:
olid
'smelly' is the definition.
(archaic)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for olid that I've seen before include "rank" , "Rank-smelling" , "Foul-smelling" , "evil-smelling" .)