According to the testimony of St. Irenaeus, Cletus is to be identified with Ancletus, who governed the Church after Linus and before Clement. Oh his actions we know nothing beyond the notice in the Liber Pontificalis to the effect that he embellished the tombs of the Princes of the Apostles, and that he himself was buried in the Vatican. The fact that Cletus was raised to the supreme pontificate, whilst the immediate disciples of Peter and Paul were still living, testifies to his great merits.
Even more obscure is the history of Pope Marcellinus, concerning whom the strangest legends were circulated from very early days. According to certain apocryphal writings, dating from the time of the disputes of Symmachus, he had formerly offered incense to idols, but had afterwards expiated his apostasy like Peter, and had voluntarily exposed himself to martyrdom. St. Cletus lived in the first Century; St. Marcellinus at the end of the third.
(Source: Fr. Lasance, The New Roman Missal)