Armoured protection for the post (4)
I believe the answer is:
This might be a double definition.
'armoured protection' is the first definition.
'mail' can be an answer for 'protection' (mail is a kind of protection). I am not sure about the 'armoured' bit.
'post' is the second definition.
(I know that post can be written as mail)
'for the' acts as a link.
Can you help me to learn more?
(Other definitions for mail that I've seen before include "Letters and parcels" , "some self-defence material" , "American post from Lima" , "Post in America" , "Armour of interlinked metal rings" .)