Tony has some great suggestions there.
Here's a klugey approach that doesn't require plugins.
In firefox, you can right select the text and "view source". Then copy the selected source. Paste into your TW tiddler.
NOW, the problem is that all of Wikipedias links are relative. You can try, to change the base at the top of your tiddler:
This will change the base reference and make the links functional. This is a kludge. I don't think the "base" tag is actually supposed to work outside the head tag, but it seems to on FF.
However, it will also change the base reference to any tiddlers that have relative links to your local image files, for example. A lot of people don't seem to use these, so that may not be a problem. Fortunately, it doesn't change internal TW linking because TW uses it's own TW mechanism. It will also change the base path of other links if you are clipping from some other source that uses relative links. You can add a base tag to the new tiddler, but whichever tiddler is at top will win the base name war. The base name is also only in effect when the tiddler is open. (I think).
-- Mark