Complex sentence: Zemlinsky had a valuable supporter in Johannes Brahms; it was he who recommended the younger composer's "Clarinet Trio" (1896) to the Simrock company for publication.
Split into simpler sentences: Zemlinsky had a valuable supporter in Johannes Brahms. Brahms, impressed with Zemlinsky's music, recommended the younger composer's "Clarinet Trio" (1896) to the Simrock company for publication.