Of fire (7)
I believe the answer is:
igneous
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of fire</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for igneous that I've seen before include "(Rock) set from lava" , "(Of rock) originating in molten state" , "Like fire - type of rock" , "Of fire, fiery" , "Of rocks, volcanic" .)