Armour in the post (4)
I believe the answer is:
This clue is a double definition.
'armour' is the first definition.
(I know that mail is a type of body armor)
'the post' is the second definition.
(synonyms)
'in' acts as a link.
(Other definitions for mail that I've seen before include "Post or body armour" , "Armour of interlinked metal rings" , "Armour; posted items" , "Post in America" , "Letters and parcels" .)