Possible Clues:

• First #1 hit for the Commodores
• 1978 hit for the Commodores
• 1978 Commodores hit
• 1978 Commodores hit whose title sounds mathematical (... letters 10-14)
• 1978 hit whose title is sung after "You're once, twice," ... and a hint to the starts of 17-, 28- and 50-Across
• Song title line after "once, twice"
• Song title line after 'once, twice'
• 1978 No. 1 hit for the Commodores, and an alternate title for this puzzle
• Treble with a song about diamonds -- The 13 sang it
• 1978 #1 hit for the Commodores (and this puzzle's title)