Whistle blower

REF

Think of the person in charge who might signal a foul during a game.

In sports, a "ref" is short for referee—the person who blows the whistle to enforce rules. The clue plays on the literal meaning of "whistle blower," since referees use whistles to signal fouls or stop play. It’s a straightforward pun where the answer isn’t about exposing secrets but about the official who literally blows a whistle.