The setter's rich source (4)
I believe the answer is:
mine
'rich source' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mine that I've seen before include "Store" , "Donnybrook" , "Dig for minerals like coal" , "Pit; source" , "Rich source" .)