Armour for men, reportedly (4)
I believe the answer is:
'armour' is the definition.
(mail is a kind of armour)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mail that I've seen before include "Post or body armour" , "American post from Lima" , "Eg, letters" , "Post in America" , "Paper" .)