Noun clause is a dependent clause that acts as a noun in a sentence. In the given sentence above, the "Whoever is tallest" is the noun clause. It contains the subject "whoever" and the verb "is". The clause functions as the SUBJECT of the sentence.
The answer to your question would be that the noun clause in the sentence above would be the following one: Whoever is tallest. What is more, the function of this clause is to be the subject of the sentence.
A noun clause is a dependent clause that acts as a noun. Therefore, they can act as subjects, direct objects, indirect objects, predicative nominatives, or objects of a preposition.
