Metal containers? (4)
I believe the answer is:
ores
'metal containers' 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" , "Rocks or minerals containing metals" , "Bauxite, haematite etc" .)