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