Old woman one left in post (4)
I believe the answer is:
'post' is the definition.
('mail' can be a synonym of 'post')
'old woman one left' is the wordplay.
'old woman' becomes 'ma' (I've seen this in other clues).
'one' becomes 'li' (I've seen this in other clues**).
'left' shows that the letters should be reversed in order.
'li' written backwards gives 'il'.
'ma'+'il'='MAIL'
'in' acts as a link.
(Other definitions for mail that I've seen before include "Form of armour; post" , "Eg, letters" , "Post or body armour" , "Dispatch - armour" , "protective cover" .)