Possible Clues:

• Mail that isn't opened
• Job given to character to provide a means of communication
• Where to write for job requiring wit
• Requiring employment, character means to communicate
• 24 and 10 across brings word of travellers
• It's sent without an envelope
• Contest entry, perhaps
• See 39- and 42-Across
• Deltiologist's purchase
• Bringer of message: vehicle on street has gone into school