Possible Clues:

• Piece of music managed to be accepted by very good singer
• Thus piano extended old singer
• Upscale performer
• She is used to having high scores
• Heartless opera with endless song all about a person in it
• Castrato's register, perhaps
• Callas, for one
• Voice above the rest
• So quiet a run for one chorus girl, perhaps
• Singing voice in the 17-Across