Possible Clues:

• Stop being so serious
• It's the responsibility of the luminary to relax
• Relax, go blonde!
• To calm down, use lift at college
• Lose pounds on a horse? Relax!
• 'Chill!'
• ''Don't act so serious!''
• ''Relax!''
• ''Don't be so serious!''
• See 64 Across