One may be prosecuted on foot of a bill (6)
I believe the answer is:
poster
'bill' is the definition.
(I know that bill can be written as poster)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for poster that I've seen before include "Large decorative printed picture" , "Large advert" , "'Notice, placard (6)'" , "bulletin board participant" , "Town near Glasgow" .)