Delete iMovie from iCloud

To delete from iCloud a movie that was exported from iMovie, sign in to iCloud.com with your Apple I.D. and password. Click on the iCloud Drive folder. Find the video that you want to delete and select it. Click on the trash icon in the tool bar at the top of the screen.

-- Rich

Here's a link to an Apple Support bulletin that describes what happens when you delete a photo or video from iCloud Photos.

Download iCloud photos and videos - Apple Support

-- Rich

Adding to my previous post, deleting from the cloud may delete the video from all of your devices as well. So, first try it with a video that you don't care about losing.

-- Rich