Rich deposit (4)
I believe the answer is:
lode
'rich 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 "A deposit of ore, say gold" , "Deposit of valuable ore running through other rocks" , "Vein containing metal ore" , "Rich source" , "mine to some extent" .)