Mercurial, changeable (8)
I believe the answer is:
volatile
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">mercurial</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">changeable</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for volatile that I've seen before include "Fickle; evaporating easily" , "Liable to unpredictable change" , "Subject to sudden changes" , "apt to change" , "Changing from one mood to another, lively" .)