Piano doesn't fit with "o" from mesomorph; "g" from guarani, or "n" from leprechaun. The answer "organ" fits in the crossword and with "Bach", but I'm not aware of Mozart or Brahms being famous for any organ compositions although Mozart wrote 3 organ sonatas and Brahms wrote 11 choral preludes. Their speciality was the piano. It seems like a poor clue.