Old name for sulphur (9)
I believe the answer is:
brimstone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">old name for sulphur</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)