Hi, deirdre,
Ziggywilmot is correct. When you delete a clip from the Original Media folder of the iMovie Library the deleted clip will be disabled in all projects that use it. The projects themselves do not contain the original clip but rather refer to the clips contained in the iMovie Library. As ziggywilmot says, if you share out the project (i.e, to your Desktop or to Theater), the shared out movie becomes a standalone, no longer dependent on the clips in the iMovie library. You can still edit the original project if you haven't deleted the clips from the library, but the ability to edit the shared out movie itself will be very limited.
The best way to deal with storage problems is to store your iMovie library on an external hard drive that is formatted Mac OS Extended (Journaled). If you reformat your external drive be aware that reformatting will erase all data stored on the drive so be sure to back up any data that you want to keep.
Here's a link that will explain how to use a library on an external drives to store projects:
https://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7
-- Rich