Volcanic glass (8)
I believe the answer is:
obsidian
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">volcanic glass</span>' is the definition.<br/ ><span class="explanation-format__explanation">(obsidian is a kind of volcanic glass)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for obsidian that I've seen before include "Dark volcanic glass" , "Dark volcanic rock" , "A glassy volcanic rock" , "Glass bottle may be in this" , "Glassy black igneous rock" .)