Vowelless rebuke

TSK

Think of the sharp sound you might make when mildly disapproving—no vowels needed.

This clue plays with the idea of a "rebuke"—a sharp disapproval sound like "tsk," which people make by clicking their tongue. The word "vowelless" hints that the answer has no vowels, so you drop the "A," "E," "I," "O," or "U" from the sound you’d expect. That leaves you with just the consonants: "T," "S," and "K," spelling out the answer.