Guy auditioned in armour (4)
I believe the answer is:
'armour' is the definition.
(I know that mail is a type of coat of mail)
'guy auditioned' is the wordplay.
'guy' becomes 'male' (I've seen this in another clue).
'auditioned' indicates a 'sounds like' (homophone) clue (I've seen 'of audition' mean this (audition can mean hearing)).
'male' sounds like 'MAIL'.
'in' is the link.
(Other definitions for mail that I've seen before include "Post or body armour" , "Posted letters" , "Flexible armour" , "Form of armour; post" , "Dispatch - armour" .)