Hello, I have two answers for you depending on how you meant to ask your question!
Q. "A second-year student is called a sophomore." Phrase or clause?
A. It's a clause because it contains a subject and predicate. Also, it can stand alone as a sentence, but phrases can't!
Q. A second-year student is called a sophomore, phrase, or clause?
A. Second year students are called sophomores!
I hope that helped! ✿