Lightweight metal (9)
I believe the answer is:
aluminium
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lightweight metal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aluminium that I've seen before include "Silvery metallic element" , "Light metal" , "Lightweight metal" , "Al" , "Metallic element found in bauxite" .)