Possible Clues:

• Elvis' music
• 1950s' pop music
• What Sisyphus has and does for entertainment
• Music genre from late 1950s
• Music genre that evolved in the '50s
• Music genre since the '50s
• Type of pop music
• One word that precedes 'hard,' one that follows it
• Subject of a 1950s 'revolution'
• Chuck Berry's music