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