Missy took over Clara's mind. Instead of her short leisure trip with the Doctor, Clara obeyed Missy's commands Clara saw Missy, pushed her body to the ground, hiked up her skirt and straddled her.
No matter what Clara did with Missy, Clara couldn't be satisfied. Missy mocked her, grinning while she should feel she should be basking in her orgasm.
Missy felt bad for Clara, however, and allowed Clara to feel an orgasm.
While Clara was recovering from her orgasm, Missy stood up and walked in front of her, as if to say Clara would be always under her control.