Post: old woman has one on line? (4)
I believe the answer is:
'post' is the definition.
('mail' can be a synonym of 'post')
'old woman has one on line?' is the wordplay.
'old woman' becomes 'ma' (I've seen this in other clues).
'has' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'on' says to put letters next to each other.
'line?' becomes 'l'.
'ma'+'i'+'l'='MAIL'
(Other definitions for mail that I've seen before include "Send message" , "Armour of interlinked metal rings" , "Post in America" , "Letters - armour" , "some self-defence material" .)