Metal-yielding rocks (4)
I believe the answer is:
ores
'metal-yielding rocks' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ores that I've seen before include "Sources of metal" , "Metal-bearing minerals" , "Mined minerals" , "Bauxite, haematite etc" , "Rocks or minerals containing metals" .)