Lightest metallic element (7)
I believe the answer is:
lithium
'lightest metallic element' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lithium that I've seen before include "Lightest alkali metal" , "Light metal element" , "Soft silvery element" , "Lightest-known metal" , "Chemical element (Li), used in batteries" .)