Possible Clues:

• Holiday message carrier
• Vacationer's correspondence
• Bearer of message whose view is one-sided?
• One sent home after spade, maybe
• One-sided view from a correspondent?
• Communication from witty type after appointment
• Jack is not the first from 24 down bringing news of the travellers
• Mail that doesn't need an envelope, and where both words of 12-, 20-, 34-, and 51-Across can be found?
• Stop silly club maybe providing old-fashioned communication
• A word from traveller and station wagon in America departs