Bit of quartz in coppery metal (4)
I believe the answer is:
zinc
'metal' is the definition.
(zinc is a kind of metal)
'bit of quartz in coppery' is the wordplay.
'bit of' indicates a hidden word.
'ZINC' can be found hidden inside 'quartz in coppery'.
(Other definitions for zinc that I've seen before include "trace element" , "Mineral" , "White metallic element, symbol Zn" , "coat for protection" , "Blue-white metallic element, atomic number 30" .)