Lightest alkali metal (7)
I believe the answer is:
lithium
'lightest alkali metal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lithium that I've seen before include "Silver-white metal (symbol: Li)" , "Light metallic element" , "Light metal element" , "Lightest-known metal" , "Lightest metallic element" .)