Yeast, mold and mushrooms

FUNGI

Think of the kingdom that includes decomposers like shelf-like growths on trees or the organisms that help make bread rise.

This clue points to "FUNGI" because yeast, mold, and mushrooms are all types of fungi—a group of organisms that aren’t plants or animals. The clue lists common examples to nudge you toward the broader category they belong to, making it a straightforward definition without any hidden wordplay or tricks. If you know basic biology, this one clicks right away.