Feature about a post requiring armour (5,4)
I believe the answer is:
chain mail
'armour' is the definition.
(chain mail is a kind of armour)
'feature about a post' is the wordplay.
'feature' becomes 'chin' (facial feature).
'about' is an insertion indicator.
'post' becomes 'mail' (synonyms).
'chin' enclosing 'a' is 'chain'.
'chain'+'mail'='CHAIN-MAIL'
'requiring' acts as a link.
(Other definitions for chain mail that I've seen before include "Protective clothing" , "Type of armour" , "trouble by post" , "Armour with tiny metal rings" , "defensive link-up" .)