Bill as user of letterbox? (6)
I believe the answer is:
poster
I believe this is a double definition.
'bill' is the first definition.
(I know that bill can be written as poster)
'user of letterbox?' is the second definition.
(someone posting a letter)
'as' acts as a link.
(Other definitions for poster that I've seen before include "Paper advertisement for topers" , "Large printed bill" , "Large decorative printed picture" , "Large advert" , "Large sheet of paper advertising something" .)