。
It used to be believed she was a former movie star.
Literal
She [topic-は] before movie-star was [quotative-と] was-being-thought.
Layered tense and modality. 以前 (formerly) modifies the inner clause — she was a movie star before. The と quotes 映画スターだった as the content of belief, and 思われていました is the polite past progressive of the passive 思われる ('be thought') — 'was being thought.' The double pastness places both the supposed stardom and the past belief about it firmly in the historical past, with the implication that the belief has since changed (perhaps disproven, perhaps simply faded).