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