Vein fill (3)
I believe the answer is:
ore
'vein fill' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ore that I've seen before include "Mineral - Scandinavian money" , "Mineral-bearing material" , "Foreign currency" , "Metal-bearing rock or mineral" , "It's mined for minerals" .)