Element used in batteries (7)
I believe the answer is:
lithium
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">element used in batteries</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)