The letters one left for mother (4)
I believe the answer is:
'the letters' is the definition.
(I've seen this before)
'one left for mother' is the wordplay.
'one' becomes 'li' (I've seen this in other clues**).
'left' says the letters should be written backwards.
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'mother' becomes 'ma' (ma is a kind of mother).
'li' back-to-front is 'il'.
'il' put after 'ma' is 'MAIL'.
(Other definitions for mail that I've seen before include "Post or body armour" , "Post in America" , "Letters and parcels" , "letter?" , "Posted letters" .)