Vein of metal ore (4)
I believe the answer is:
lode
'vein of metal ore' is the definition.
(I've seen this before)
This is all the clue.
(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" .)