quicksilver (7)
I believe the answer is:
mercury
'quicksilver' is the definition.
(I know that quicksilver can be written as mercury)
This is the entire clue.
(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" .)