Silvery-white metallic element (9)
I believe the answer is:
potassium
'silvery-white metallic element' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for potassium that I've seen before include "Soft metal, symbol K" , "Soft metallic element" , "Soft reactive metal" , "Silvery metallic element" , "... is its atomic number: symbol K" .)