The phrase which is an adjective clause in the sentence is " who at first only wanted to study and play the piano."
An adjective clause which is also called an adjectival clause, is a form of dependent clause intended to modify a noun in a sentence. It functions as an adjective even though it is composed of a group of words instead of just one word. In the case of an adjective clause, all the words work together to describe the noun or pronoun.
