According to tradition, Clement of Rome knew some of the Apostles. And there’s no particular reason to doubt that he may have known one or more of the Apostles.
Now, according to Catholic and Orthodox apologetics, if a subapostolic father knew one or more of the Apostles, then this creates the presumption that his teaching preserves and passes along Apostolic doctrine. He’s a custodian and a conduit of Apostolic tradition. We may safely assume that he’s repeating what he heard from the lips of an Apostle.
But I can’t help noticing that this pious presumption comes to an abrupt and premature halt when we arrive at the science of Clementine ornithology:
“Let us consider that wonderful sign [of the resurrection] which takes place in Eastern lands, that is, in Arabia and the countries round about. There is a certain bird which is called a phoenix. This is the only one of its kind, and lives five hundred years. And when the time of its dissolution draws near that it must die, it builds itself a nest of frankincense, and myrrh, and other spices, into which, when the time is fulfilled, it enters and dies. But as the flesh decays a certain kind of worm is produced, which, being nourished by the juices of the dead bird, brings forth feathers. Then, when it has acquired strength, it takes up that nest in which are the bones of its parent, and bearing these it passes from the land of Arabia into Egypt, to the city called Heliopolis. And, in open day, flying in the sight of all men, it places them on the altar of the sun, and having done this, hastens back to its former abode. The priests then inspect the registers of the dates, and find that it has returned exactly as the five hundredth year was completed.”