What do you call a noun clause function as subject complement?

A noun clause functioning as a subject complement is simply called a subject complement clause.

Here's why:

* Subject complement: A subject complement is a word or phrase that renames or describes the subject of a sentence. It usually follows a linking verb (like "be", "seem", "appear", etc.).

* Noun clause: A noun clause is a clause that functions as a noun. It can act as the subject, object, or complement of a sentence.

Example:

* The truth is that she is innocent.

In this sentence:

* "The truth" is the subject.

* "is" is the linking verb.

* "that she is innocent" is the noun clause functioning as the subject complement because it renames the subject "The truth."

Therefore, "that she is innocent" is a subject complement clause.