A fragment of quartz in cuprous metal (4)
I believe the answer is:
zinc
'metal' is the definition.
(zinc is a kind of metal)
'a fragment of quartz in cuprous' is the wordplay.
'a fragment of' says the answer is hidden in the clue (I've seen this in other clues).
'ZINC' is hidden in the letters of 'quartz in cuprous'.
(Other definitions for zinc that I've seen before include "Metal used in brass" , "Metal alloyed with copper to make brass" , "Antioxidant element" , "trace element" , "Bluish-white lustrous metallic element" .)