Identify the clause "That you were the first one to cross the finish line" as a noun, adjective, or adverb.

That you were the first one to cross the finish line has now been verified.

The clause "That you were the first one to cross the finish line" is functioning as a noun in this sentence, serving as the subject of the sentence "has now been verified."

The clause "That you were the first one to cross the finish line" functions as a noun. It is the subject of the sentence "has now been verified."