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