Armour written about in Malachi (5,4)
I believe the answer is:
chain mail
'armour' is the definition.
(chain mail is a kind of armour)
'written about in malachi' is the wordplay.
'written about' indicates anagramming the letters (letters moved about before being rewritten).
'in'+'malachi'='inmalachi'
'inmalachi' anagrammed gives 'CHAIN-MAIL'.
(Other definitions for chain mail that I've seen before include "Flexible body armour" , "Type of armour" , "circular letters?" , "trouble by post" , "Armour of small metal links" .)