Complex sentence: Copland's compositions in the early 1920s reflected the modernist attitude that prevailed among intellectuals, that the arts need be accessible to only a cadre of the enlightened and that the masses would come to appreciate their efforts over time.
Split into simpler sentences: Copland's compositions in the early 1920s reflected the modernist attitude that prevailed among intellectuals, who thought of themselves as a small vanguard for the masses. In their view, music and the other arts need be accessible to only a select cadre of the enlightened, since the masses would only come to appreciate their efforts over time.