Possible Clues:

• It's the sack for a public employee
• Result of operation of prison sewer?
• Trouble facing book in publication in which letters appear?
• Something for sewer that's been sent down
• This clue's answer might contain more than seven letters
• *Sack with letters
• Pouch for post
• Sack on a red-white-and-blue truck, which can hold the ends of 21- and 54-Across and 3- and 35-Down
• Postman's burden
• Postholder’s speciality getting to the bottom of motorway trouble