Possible Clues:

• Some surprise hit songs, and what 1-, 21-, 37-, and 53-Across all have?
• Backs of singles
• Unlikely hits
• 'I Am the Walrus' and 'Revolution,' for two
• Songs considered less catchy also lacking energy
• Also leaving Spain, making tracks
• Some surprise hits
• “Revolution” and “Unchained Melody,” for two
• 45-era's lesser songs
• Not the hits