Clifford Danner was one of Mike Ross's friends and as a student, was wrongfully imprisoned for a murder that he did not commit. He was found guilty when D.A. Cameron Dennis buried evidence that, if disclosed, would have proved him innocent.
He was sentenced to 16 years in jail. However, after serving only 12 years, he was released after Harvey Specter proved his innocence and exonerated him. After that, he was seen in "Endgame" working in a restaurant (possibly waiter and bartender).
In the season 5 episode "Tick Tock," Mike goes to his house to ask him to testify at his trial, only to see his mother Gloria there, who tells Mike that he (Clifford) was shot and killed three weeks ago. Mike then has his mother testify instead, and she tells the jury that Clifford had been killed in a robbery.