The woman, one of seven alleged victims of indecent assault by Mr Clifford, told London's Southwark Crown Court she was about 18 when she was assaulted by the PR guru in the early 1980s.
She said she was leaving a dance audition in a London nightclub when Mr Clifford, 70, stopped her and claimed he was acting for James Bond producer Cubby Broccoli, who was doing screen-testing for the film Octopussy.
The woman, who cannot be named, said he made a call from a public phone box before calling her over.
She told the court the person on the phone said he was Broccoli. "He said you need to do doing something before we can agree to the screen test. He asked me if Max was circumcised and asked me to look. I thought it was a joke. I kind of froze. Then I realised I was stuck."
The woman then broke down in tears. After composing herself, she told the court Mr Clifford took her by the hand into the male toilets.
She said he pushed her into a cubicle before making her perform a sex act on him. "He said something very cocky like, no-one is going to believe me, or 'Who's going to believe that?'"
Richard Horwell, QC, defending Mr Clifford, suggested the alleged assault "simply did not happen".
She replied: "I wish I could say that was true."
Mr Clifford, from Hersham in Surrey, is accused of 11 counts of indecent assault. He denies all of the charges and the trial continues.