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