Complex sentence: Leslie Howard, however, could not play the violin, so a professional violinist--who also bore a striking resemblance to Leslie Howard--was brought in to teach him proper violin posture and bowing technique.
Split into simpler sentences: Leslie Howard, however, could not play the violin. As a result a professional violinist, who also held a striking resemblance to Leslie Howard, was brought in to teach proper violin posture and bowing technique.