Hi Kiwistack,
Thank you for participating in the Apple Support Communities. We understand you'd like to delete files on iCloud Drive in order to free up space, but you want to make sure you still have a. copy of the file on your Mac. You'll just want to make sure you have a copy of the file on your Mac in a location that is not syncing with iCloud Drive. For example, if you have Desktop and Documents added to iCloud Drive, you'll want to make sure those files are stored on your Mac in a location that is not in Desktop and Documents. This article explains more about this and how to check if Desktop and Documents is turned on: Add your Desktop and Documents files to iCloud Drive - Apple Support.
Take care.