"No acting up!"

BEGOOD

Think of a phrase a parent might say to a child before leaving them alone, emphasizing good behavior.

The clue "No acting up!" is a playful way to say "behave yourself," which directly hints at the answer "BEGOOD." It’s a simple phrase parents or teachers might use to tell someone to stop misbehaving and act properly. The wordplay is straightforward—no tricks or hidden meanings, just a common expression turned into a concise crossword answer.