Writer of a letter to the post office (3)
I believe the answer is:
poe
'writer' is the definition.
(Edgar Allan Poe)
'letter to the post office' is the wordplay.
'letter' becomes 'e' ().
'to' means one lot of letters go next to another (I've seen this in other clues).
'the post office' becomes 'po' (abbreviation).
'e' put after 'po' is 'POE'.
'of a' is the link.
(Other definitions for poe that I've seen before include "Edgar Allan --, US writer" , "Author of The Murders in the Rue Morgue" , "US horror master" , "Edgar Allen, great American writer of eerie stories" , "Entrance rooms; seeks support from" .)