Metal in brass (4)
I believe the answer is:
zinc
'metal in brass' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for zinc that I've seen before include "Sivery-white metal" , "Silvery-white metal" , "Mineral" , "Blue-white metallic element, atomic number 30" , "White metallic element usedin alloys" .)