Crossword Genius

One who mails a bill (6)

Ross

I believe the answer is:

poster

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'bill' is the definition.
(I know that bill can be written as poster)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for poster that I've seen before include "bulletin board participant" , "Bill or placard" , "Paper advertisement for topers" , "Person who sends" , "surfer who leaves messages on boards?" .)

I've seen this clue in The Globe and Mail.
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