Possible Clues:

• Word sung after the ball drops
• End of a New Year's Eve song title
• Champagne-fueled song finale
• “Ago”, as in the title of a well-known Scottish song
• Famous Jan. 1 song word
• Auld Lang —
• Start-of-year song ender
• “Auld Lang ___”
• 'And we'll tak' a cup o' kindness yet / For auld lang ___'
• Word in a year-end song