I notice that when I add and fx on a clip it takes (ofcourse) harddrive-space. When I change the fx slightly and render again, it takes (probably ofcourse) again the same amount of space. Is it possible to check or free-up space from rendered clips that aren't really used anymore?

I hope you understand the question....

 Yes,

Within the media tool select your project and look at precomputes only, then when the bin opens with all your precomputes go to the hamburger menu and click select unreferenced clips, then hit delete. (That's how it works on a PC)