Possible Clues:

• Invoice, more than twice as high
• What boys sing is in three parts
• Seems the invoice is more than twice as high
• Three times higher than bass
• Singer; in three parts
• High voice; occurring three times
• More than twice as high, by the sound of it (6)
• Chance for a singer?
• High-pitched or soprano, instrument or boy's voice
• A clef