It's in vein (4)
I believe the answer is:
lode
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it's in vein</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lode that I've seen before include "Rich source" , "Vein containing metal ore" , "Vein of metal in the earth" , "Vein of metallic ore sounds heavy" , "mine to some extent" .)