Post that once provided personal protection (4)
I believe the answer is:
This might be a double definition.
'post that once provided personal' is the first definition.
'mail' can be an answer for 'post' (synonyms). I am unsure of the remainder of the definition.
'protection' is the second definition.
(mail is a kind of protection)
Can you help me to learn more?
(Other definitions for mail that I've seen before include "Chain armour" , "Letters and parcels" , "some self-defence material" , "Armour; posted items" , "Send message" .)