Vein of metal ore (4)
I believe the answer is:
lode
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vein of metal ore</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 lode that I've seen before include "Vein of metal ore in surrounding rocks" , "A deposit of ore, say gold" , "mine to some extent" , "Rich source" , "Deposit of valuable ore running through other rocks" .)