Related Clues:

• Singing sounds
• Sings at a high level?
• Is a rat
• Acts the stool pigeon
• Spills the beans
• Emulates a 43-Across
• Sings Swiss-style
• Sings in peak form?
• Displays peak singing condition?
• Serenades Heidi, maybe
• Sings in the Alps
• Falsetto songs
• Alpine refrains
• Mountain airs?
• Mountain valley sounds