Complex sentence: In 1967, Burroughs became more serious about Scientology, taking several courses and becoming what the Church of Scientology calls a "clear" and in the 1970s, while living in London, what the organization calls an "Operating Thetan".
Split into simpler sentences: In 1967, Burroughs became more serious about Scientology, taking several courses and becoming what the Church of Scientology calls a "clear". In the 1970s, while living in London, he progressed to the level of "Operating Thetan".