Complex sentence: Fonda's communications, as well as those of her husband, Tom Hayden, were intercepted by Britain's Government Communications Headquarters (GCHQ), which under the UKUSA Agreement, intercepted data on Americans were sent to the U.S. government.
Split into simpler sentences: Fonda's communications, as well as those of her husband, Tom Hayden, were intercepted by Britain's Government Communications Headquarters (GCHQ). Under the UKUSA Agreement, intercepted data on Americans were sent to the U.S. government.