Complex sentence: She then joined the Secret Intelligence Service (MI6) but when Kim Philby, later to be exposed as a double agent, became her boss he reduced her investigative work because he feared she might uncover his treachery.
Split into simpler sentences: She then joined the Secret Intelligence Service (MI6). When Kim Philby, later to be exposed as a double agent, became her boss he reduced her investigative work because he feared she might uncover his treachery.