Complex sentence: His first client was the then-unknown screenwriter Bo Goldman, whose script for "One Flew Over the Cuckoo's Nest" won the Academy Award for "Best Adapted Screenplay" in 1975.
Split into simpler sentences: His first client was the then-unknown screenwriter Bo Goldman. Shortly after signing with Stiefel, Goldman wrote the script for "One Flew Over the Cuckoo's Nest," which won the Academy Award for "Best Adapted Screenplay" in 1975.