Vein of ore (4)
I believe the answer is:
lode
'vein of ore' is the definition.
(lode is a vein of metal ore)
This is the entire clue.
(Other definitions for lode that I've seen before include "Vein containing metal ore" , "Vein of metal ore in surrounding rocks" , "Rich source" , "mine to some extent" , "Deposit of valuable ore running through other rocks" .)