Planet nearest the sun (7)
I believe the answer is:
mercury
'planet nearest the sun' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mercury that I've seen before include "Metal in thermometer or barometer" , "In Roman myth, the messenger of the gods" , "Atomic number 80" , "Quicksilver" , "One associated with Queen" .)