Sulphur (9)
I believe the answer is:
brimstone
'sulphur' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brimstone that I've seen before include "Fire partner" , "Sulphur; butterfly" , "Old name for sulphur" , "Sulphur (when associated with hellfire)" , "hellish punishment" .)