Volcanic molten rock (4)
I believe the answer is:
lava
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">volcanic molten 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 "Outflow from volcano" , "Unstoppable red tide" , "Molten matter from volcano" , "Discharge from volcano" , "Volcanic emission" .)