Molten volcanic rock (4)
I believe the answer is:
lava
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">molten volcanic rock</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 lava that I've seen before include "Volcanic outflow" , "Liquid rock" , "Volcanic outpouring" , "Unstoppable red tide" , "Hot flowing rock" .)