Metal deposit (4)
I believe the answer is:
lode
'metal deposit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lode that I've seen before include "Deposit of valuable ore found in surrounding rocks" , "Deposit of valuable ore running through other rocks" , "Rich source" , "Vein of metal ore in surrounding rocks" , "Vein of metal in the earth" .)