Clark's first exposure to the music of Arnold Schoenberg was at a performance of his symphonic poem Pelleas und Melisande on 31 October 1910 at a concert by the Gesellschaft der Musikfreunde in Berlin. He was profoundly impressed by this music, and he was introduced to Schoenberg and Anton Webern after the concert. He became Schoenberg's champion and, along with people like Artur Schnabel, Ferruccio Busoni, Oskar Fried and others, he convinced him to move from Vienna to Berlin, because of the greater opportunities and contacts there. From 1910 until 1912 Clark studied with Schoenberg in Berlin, the only British student he ever had. At first Clark was his sole pupil, then he was joined by Eduard Steuermann, and others later. Clark arranged for Schoenberg to give a series of ten public lectures at the Stern Conservatory ("10 Lectures on the Aesthetics of Music and the Rules of Composition"), which he also attended. He helped raise funds for Schoenberg, and played a part in having his Five Pieces for Orchestra published in a cheap edition, which sold out quickly. One of the purchasers was Sir Henry Wood.
