The witness told Southwark Crown Court, London, her ex-boyfriend was once married to a woman who previously gave evidence against the 70-year-old, claiming he had assaulted her when she was 14.
Giving evidence, the woman said he told her he had been asked to "corroborate" his ex-wife's story, but in reality "nothing had happened".
Clifford is accused of 11 counts of indecent assault against seven girls and women - all of which he denies.
Asked by defence barrister Richard Horwell, QC, what her motivation for coming to court was, the witness said: "To tell the truth. Because I felt they were not telling the truth."
Questioned about what her ex-boyfriend told her about the alleged victim, she said: "That when they first married she had told him all about this incident.
"She and a friend were outside a club in London and that Max Clifford had offered them a lift home but that nothing had happened."
Clifford, who listened to the evidence with the aid of a hearing loop, later smiled broadly as his brother, Harold, entered the court room to give evidence as a defence witness.
Mr Clifford, 77, said he suspected his sibling had an affair with one woman, but did not know of any others he had extra-marital relationships with.
He added he would not have expected him to speak to his family of such matters.
The trial continues.