Related Clues:

• Diamond V.I.P.'s
• Field experts
• Diamond quartet, briefly
• Boo targets, often
• Baseball officials
• Fight stoppers
• Umpires
• They know when you're out
• Football officials
• Zebras, in sports slang
• They whistle while they work
• Arena arbiters
• Flag men
• Court judges
• Some whistle-blowers, briefly