Volatile or lively (9)
I believe the answer is:
mercurial
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">volatile</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">lively</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for mercurial that I've seen before include "Volatile" , "lively" , "rapidly changing" , "Changing unpredictably" , "Sprightly" .)