Quicksilver (7)
I believe the answer is:
mercury
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quicksilver</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that quicksilver can be written as mercury)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mercury that I've seen before include "one in distant orbit?" , "Metal in thermometer or barometer" , "First rock from the Sun - Hg" , "Planet nearest the sun" , "Lead singer" .)