Possible Clues:

• Simon & Garfunkel song
• Simon & Garfunkel hit, and hint to the starts of 17-, 24-, 48- ... and 35-Across, too!
• Simon & Garfunkel song about emotional detachment
• Final cut on the "Sounds of Silence" album
• Final cut on the 'Sounds of Silence' album
• Song that starts 'A winter's day in a deep and dark December'
• Simon and Garfunkel song
• 1966 hit from the album 'Sounds of Silence'
• 1966 Simon & Garfunkel hit
• 1966 Simon and Garfunkel hit