Possible Clues:

• Athlete's best
• Athlete’s peak performance
• 'It's only ___'
• It's what we expect best footballers to bring to Dublin versus Cork
• 'War's ___': Cowper
• "It's only ___!"
• Where Do You Go? they inquired
• Top performance, informally
• Peak performance, informally
• One's best effort, in sports slang