Complex sentence: The music video itself does not feature Emeli Sande, who does provide the actual vocals for the chorus, but for reasons unknown is replaced by an alternative woman who appears throughout the video miming along to the pre-recorded vocals.
Split into simpler sentences: The music video does not feature Emeli Sande, who provides the actual vocals for the chorus on the recording. She is replaced by an alternative woman who appears throughout the video miming along to the pre-recorded vocals.