。
She promised to clean my room.
Literal
She [topic-は] my room [object-を] clean [into-に] make [quotative-と] promised.
The Xを Yにする pattern ('make X into Y') here turns 部屋 ('room') into きれい ('clean') — 'clean the room.' This is then quoted with と and attached to 約束する ('to promise') as the content of the promise. Note that 約束する takes its content with と (the quotative), not directly: 私は何々と約束した ('I promised that ~'). The verb 約束する is built from the noun 約束 ('promise, appointment, commitment') + する.