。
She was cleaning the house in preparation for the party.
Literal
She [topic-は] party [in preparation for-に備えて] house [genitive-の] cleaning [object-を] was-doing.
に備えて is built from the te-form of 備える ('to prepare against, equip oneself for') and gives a slightly more thorough, defensive shade than the simpler ために. The connotation is 'making ready against' something coming — used for parties, exams, disasters alike. 家の掃除 ('house cleaning') is an example of the productive [noun]の+nominalised-verb pattern that turns an action into a noun phrase by attaching の. The ~ていた past progressive sets the moment of observation while the cleaning was already underway.