Possible Clues:

• Eavesdrop
• Work on The Wire with snoop
• What spies often do
• Eavesdrop, though line isn't rigged
• Work with taps, maybe
• Eavesdrop on
• That's all you can do with the radio
• 35 across to what's on the air (6, 2)
• Overhear conversation as on phone (6, 2)
• A rising number follow tip and hear private conversation