Complex sentence: Clarke was not an accomplished musician prior to joining the Byrds--he had never played drums and, after joining the Byrds, not having a drum set, practiced on cardboard boxes and a tambourine.
Split into simpler sentences: Clarke was not an accomplished musician prior to joining the Byrds. He had never played drums and, after joining the Byrds, not having a drum set, practiced on cardboard boxes and a tambourine.