This tells us that head covering was practiced at the time of Clement, and that is what David is trying to show us.
So, using church history is useful, but it has to be used carefully. Just because it can be used to support one point does not mean the same quote can be used to support another.
As time goes on, we're going to provide some examples of people in Church history who taught about head covering. Hopefully we'll read them carefully enough to tell you what they actually said. But we'll be providing you with links to their writings so you can check, and we suggest you have a Bible nearby, also, because while they all agreed that men must cover their heads and women must uncover their heads when praying or prophesying, there are some differences in the details.
Other Resources.
1. If you're interested, buy David's book, Headcovering Throughout Christian History. It's worth your time.

2. Aaron Shafovaloff has set up a website looking at a variety of historical texts about head covering:
3. For the Tweeters among you, Aaron has set up a Twitter account with historical quotes: