Complex sentence: Martin was featured in the 1941 Marx Brothers film (their last for MGM), "The Big Store", in which he played a singer and performed "Tenement Symphony", which was written by Hal Borne who became his long-time musical director.
Split into simpler sentences: Martin was featured in the 1941 Marx Brothers film (their last for MGM), "The Big Store". In it, he played a singer and performed "Tenement Symphony", which was written by Hal Borne who became his long-time musical director.