Possible Clues:

• The grapevine
• How rumors spread
• Oral
• Informal oral communication
• What rumors are spread by ... or a hint to the ends of 16-, 29-, 35-/37- and 45-Across
• Oral means of relaying information
• Spoken communication
• The grapevine, so to speak
• Minnie's promise, as 58-Across would say
• Free way to create buzz