Possible Clues:

• It's the sack for a public employee
• Trouble facing book in publication in which letters appear?
• Postholder’s speciality getting to the bottom of motorway trouble
• *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
• Postal tote
• Armour, black and silver, generating a measure of response?
• Sack for carrying (American) letters