Metal bearers (4)
I believe the answer is:
ores
'metal bearers' 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 "Rocks or minerals containing metals" , "Metal-bearing minerals" , "Bauxite, haematite etc" , "Sources of metal" , "Mineral deposits" .)