Surface magma (4)
I believe the answer is:
lava
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">surface magma</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lava that I've seen before include "Molten rock expelled by a volcano" , "What volcano ejects" , "Obsidian" , "Hot molten rock" , "Volcanic emanation" .)