Molten matter (5)
I believe the answer is:
magma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">molten matter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for magma that I've seen before include "Molten rock in the earth's crust" , "hot stuff" , "That's hot!" , "crust's content" , "Hot fluid rock" .)