It's in vein (4)
I believe the answer is:
lode
'it's in vein' 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 "Rich source" , "Vein containing metal ore" , "Vein of metal in the earth" , "Vein of metallic ore sounds heavy" , "mine to some extent" .)