"Mm-hmm"

YEP

This casual two-letter affirmation often follows a question like "Got it?" and sounds like the start of "yellow.

Mm-hmm" is a casual sound people make to mean "yes" or agreement, just like "yep." The clue plays on how both are informal ways to say the same thing—one’s a noise, the other’s a word. Since crosswords often swap sounds for letters, the answer fits as the short, written version of that nodding response. It’s a straightforward match where the meaning does all the work.