Clement of Rome was an elder in in the late 1st century. The Roman Catholics call him a pope (Pope Clement I), but his letter is one of the best evidences against the existence of a pope in the early 2nd century.
He wrote a letter to the Corinthians in A.D. 96, approximately 40 years after Paul wrote his two letters. He lets us know the Corinthians repented of their divisiveness, but now, a generation later, it was returning. They'd even defrocked a couple elders.
The letter is addressed from the Church of Rome. Clement's name is not on it. Since the 2nd century, however, Christians have been attributing it to Clement, and so it is known as First Clement.
It's a beautiful letter, full of the Spirit of early Christianity. While some of the New Testament writings were circulating at this time, you can tell from the letter that the Old Testament is still the main "Bible" that the Church had.
Scholars think First Clement was attached to the Codex Sinaiticus, one of two complete Bibles dating from the fourth century. It's not in perfect condition, being 1700 years old, so there's pages missing.
There is another writing, called Second Clement, that is no longer attributed to Clement. Often now it will be found with the title, An Early Christian Homily. It is dated from a few decades after Clement died.
Clement of Rome is vilified here and there for believing that the story of the phoenix bird was a true story. He uses it as an example in nature of the resurrection. I can't imagine why it's a problem that he believed the poor science of his day.