Old name for sulphur (9)
I believe the answer is:
brimstone
'old name for sulphur' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brimstone that I've seen before include "Old name for sulphur" , "Yellow butterfly" , "Biblical sulphur that rained with fire on evil cities" , "It robs men (anag) - old word for sulphur" , "Fire partner" .)