Form of armour (5,4)
I believe the answer is:
chain mail
'form of armour' is the definition.
(I know that chain mail is a type of suit of armour)
This is all the clue.
(Other definitions for chain mail that I've seen before include "guard" , "Armour with tiny metal rings" , "defensive link-up" , "Cover protecting old soldier" , "Armour of small metal links" .)