Possible Clues:

• ''__ on first?''
• El DeBarge "___ Johnny"
• "__ Afraid of Virginia Woolf?"
• Villagers below Mount Crumpit
• 'Knock, knock. -- there?'
• ''Knock, knock'' response starter
• Madonna "___ That Girl"
• '-- to say?'
• "__ on first?"
• Beginning of a knock-knock answer