iMovie Timeline Not Working

Be aware that if you delete the clip from the Project Media (Control-click/Delete Project Media) you will remove it from the iMovie Library and thus from every project that may be referring to it. In other words, you will permanently be removing the clip from the iMovie system. Unless you have the clip backed up somewhere outside of iMovie, you will permanently lose your source clip. In your case it looks like you have selected a clip that is 3 hours long, which is a major clip. Because you have selected it, it is playing in the project preview screen.

For information, any time one selects a clip by clicking on it in the Project Media, the clip will play in the Preview Screen whether or not there are any clips in the timeline.

--Rich

Right-click (control-click) the clip in the Browser and select Delete from the dropdown menu.