Zn (4)
I believe the answer is:
zinc
'zn' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for zinc that I've seen before include "Sivery-white metal" , "Ingredient in brass" , "Metallic element present with copper in brass" , "it might be galvanising" , "White metallic element, symbol Zn" .)