Complex sentence: Keen to launch a solo career due to Nash's success in CSN, Clarke unexpectedly left the group in 1971, and was replaced by the Swedish singer Mikael Rickfors, who was formerly with Bamboo.
Split into simpler sentences: Keen to launch a solo career due to Nash's success in CSN, Clarke unexpectedly left the group in 1971. He was replaced by the Swedish singer Mikael Rickfors, who was formerly with Bamboo.