Cliff Tannen was a brother to Biff Tannen, along with Riff Tannen, in an alternate 1986. He was part of the notorious Tannen family mob, the fifth largest in California.
Cliff was the second oldest brother, and only appeared to exist in this alternate reality, where their father Kid Tannen never went to prison, allowing him and the Tannen family to reign supreme as a criminal syndicate for over fifty years in Hill Valley. It is unknown if he exists in the actual 1986.