There are some ways to remove a clip from the timeline. Maybe the easier one is to select the clip or clips that you want to delete, and press the "Delete" button in your keyboard.
Another way is to select the clips that you want to delete, press the right mouse button and in the context menu select option Delete Clip.
By this, you'll remove the clip from the timeline but it still appears in the "Project Tree" list. If you select the clip here and proceed as above, you'll remove it also from the project. But do not worry, it still remains physically in your disk!