* It functions as a noun: The entire clause acts as the subject of the sentence. For example, you could say: "Whoever catches the fish gets a prize." In this sentence, "whoever catches the fish" is the subject, replacing the noun phrase "the winner".
* It contains a subject and a verb: The clause has the subject "whoever" and the verb "catches."
* It starts with a relative pronoun: The clause begins with the relative pronoun "whoever," which connects it to the rest of the sentence.
Therefore, it meets all the criteria for a noun clause.