Valuable mineral (3)
I believe the answer is:
ore
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">valuable mineral</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 ore that I've seen before include "It's mined for minerals" , "Mineral deposit" , "Foreign currency" , "Material for smelting" , "it could be gold" .)