Alloy of copper and zinc (5)
'alloy of copper and zinc' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brass that I've seen before include "Alloy (associated with muck)" , "Impertinence" , "instrumental group" , "Alloy of copper and zinc" , "Nerve (but not of steel!)" .)
(Other definitions for tutenag that I've seen before include "alloy" , "zinc, loosely" .)