Here's why:
* Describes a quality: It describes a state of being free from dirt, stains, or impurities.
* Modifies nouns: It modifies nouns, like "clean clothes," "clean water," or "a clean room."
However, "clean" can occasionally be used as a noun in certain contexts:
* Short for "cleaning": You might say, "I need a clean." This means "I need to do some cleaning."
* Specific cleaning products: Sometimes, "clean" is used as a shorthand for specific cleaning products like "Windex" or "Clorox," such as "I need to get some clean."
But in general, "clean" is most often used as an adjective.