Sounds like an exam question?

And, maybe just because I am from the States, what is a greenfield?

Hi tugbrethil, greenfield means a piece of land that has not previously been built on, as opposed to a brownfield site, which means land that has been built on previously.
In the uk we try to keep as much "greenfield" or what we call "greenbelt" as we possibly can, and try to utilise as much "brownfield" as we can, as we have an ever decreasing amount of countryside to work with.
hope this explains for you, regards Derek

Are you actually faced with a site that you yourself want to level or are you talking generalities?

Hmm, Derek, it sounds, then like clearing and levelling isn't something you want to do to an area like that. Too many of our wild lands here in Arizona had chains dragged across them in the 1970's in the name of "watershed improvement". Translate: the cattle barons wanted less brush and more grass. What they got was more gullies and weeds! : /

You can always spot an essay topic - the question would need an essay for a proper answer.

Password, please don't rely on others to do your research and writing. When you are out in the real world, you will need to actually understand the whys and wherefors of this material, so you can adapt the methods to local conditions, not just spout rote answers. You owe it to yourself and the people affected by your work to put the time and mental effort into actually learning the course material, along with the critical thinking involved.
There, that's my quarterly essay against academic and intellectual laziness! ; )