Type of armour (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">type of armour</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that mail is a type of body armor)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mail that I've seen before include "Dispatch - armour" , "Chain armour" , "Armour; posted items" , "Form of armour; post" , "Letters and parcels" .)