He was a former slave of Agrippa Postumus, the grandson of Augustus, who was killed around the time Tiberius came to power. Clemens appeared claiming that he really was Postumus, and gained a significant band of followers, but was captured and executed by Tiberius. It is reputed that when he was brought before Tiberius, he was asked "How did you become Agrippa?" and he replied "The same way you became Caesar".
'The same year a certain Clemens, who had been a slave of Agrippa and resembled him to a certain extent, pretended to be Agrippa himself. He went to Gaul and won many to his cause there and many later in Italy, and finally he marched upon Rome with the avowed intention of recovering the dominion of his grandfather. The population of the city became excited at this, and not a few joined his cause; but Tiberius got him into his hands by a ruse with the aid of some persons who pretended to sympathize with this upstart. He thereupon tortured him, in order to learn something about his fellow-conspirators. Then, when the other would not utter a word, he asked him: "How did you come to be Agrippa?" And he replied: "In the same way as you came to be Caesar."' -Cassius Dio 
Robert Graves, in his novel I, Claudius, makes the curious suggestion that this person really was Postumus. This was not included in the 1976 TV adaptation, which does not include Clemens and instead explicitly shows Postumus being killed while in exile.