The phrase "clean upon" means to be completely sure or certain. There are several synonyms for this phrase that can be used interchangeably. "Absolutely certain", "completely convinced", "totally confident", "utterly satisfied", and "thoroughly persuaded" are some of the synonyms for "clean upon". When we use these phrases in our sentences or speeches, we express our level of conviction and assure the listeners of our certainty. Using synonyms for commonly used phrases keeps the language alive and vibrant and enhances the effectiveness of the message. Therefore, it is important to learn, explore, and incorporate these synonyms into our vocabulary for effective communication.