In a gun, the extremes literally are of brass (6)
I believe the answer is:
brazen
'brass' is the definition.
(thesaurus)
'in a gun the extremes' is the wordplay.
'in' is an insertion indicator.
'a gun' becomes 'Bren' (Bren is an example).
'the extremes' becomes 'az' (extreme letters of the alphabet).
'bren' enclosing 'az' is 'BRAZEN'.
'literally are of' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for brazen that I've seen before include "Insolent, barefaced" , "Bold-faced" , "Shameless and bold - resembling brass" , "Unashamedly flagrant" , "Shameless (impudence)" .)