When you delete video from a project, the video isn’t deleted from the source video (the Event) the video originates from.
To delete video from your project:
Select the clip or frame range you want to delete, and then either press the Delete key or choose Edit > Delete Selection or Edit > Delete Entire Clip.
The Edit menu appears in a light gray bar across the top of your computer screen.
To undo the deletion immediately, choose Edit > Undo Delete Selection, or press Command-Z.