There's a letter at the post office for Edgar Allan (3)
I believe the answer is:
poe
'edgar allan' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
'there's a letter at the post office' is the wordplay.
'there's a letter' becomes 'e' ().
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the post office' becomes 'po' (abbreviation).
'e' after 'po' is 'POE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for poe that I've seen before include "Fall of House of Usher author" , "US horror master" , "American author, d. 1849" , "The Raven poet" , "Edgar Allan --, US writer" .)