Sender of a bill? (6)

I believe the answer is:

poster

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

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

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for poster that I've seen before include "surfer who leaves messages on boards?" , "bulletin board participant" , "Printed advert" , "Large sheet of paper advertising something" , "Large printed bill" .)

I've seen this clue in the Evening Standard.

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