American author posts often (6)

I believe the answer is:

mailer

I'm a little stuck... Click here to teach me more about this clue!

'american' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for mailer that I've seen before include "One putting armour on" , "person at the post?" , "Poster" , "writer from America" , "Norman --, US author" .)

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app