You don't. When you cut or copy something, it goes to a specially set aside area of RAM called "the clipboard". It's just like any other area of RAM, but no other app uses it. When you paste something, what's in there gets pasted. If you cut or copy something else, what was in there gets replaced.
If you want something more manageable, use something like
Clipper - Clipboard Manager. (It doesn't use the clipboard, it keeps the data in its own space.
You can't "clear" the clipboard "out" other than copying a small blank space to it.
To copy, look for the Copy symbol, a square with 4 little squares inside it. To past, look for the two pages, one overlaying the other, or long-press in a text box when you've copied something.