The Clue Use mouthwash
The Answer RINSE
The word "rinse" fits the clue "use mouthwash" because rinsing is a common step when using mouthwash. When you use mouthwash, you swish it around in your mouth and then spit it out, which is a rinsing action. This helps to clean your mouth and freshen your breath. Therefore, "rinse" accurately describes what you do with mouthwash.