Sharp knock

RAP

Think of a quick, rhythmic sound often made with the knuckles on a door or used in music to keep time.

Sharp knock" hints at a quick, forceful tap—like the sound of someone rapping on a door. The word "rap" fits because it can mean both a sudden strike and a type of music, but here it’s the knocking sense that’s key. The clue plays on the double meaning, but the answer is straightforward: a short, sharp knock is a rap.