One component of brass (4)
I believe the answer is:
zinc
'one component of brass' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for zinc that I've seen before include "Anti-corrosion agent" , "Blue-white metal" , "Metallic element present with copper in brass" , "Type of metal" , "It is used to coat iron against corrosion" .)