Well, I just did - multiple times. The clips may exist elsewhere however they're gonna from where I want them gone from : the 'my movie' pane.
The issue I was having, was that there were so many imported clips (that I'd used only small sections of) that were sitting in the 'my movie' pane, that it was slowing iMovie down - noticeably. Also, I was struggling to find the last imported movie in the 'my movie' pane as it was so crowded, and couldn't figure out how to remove the imported clips as I'd previously been able to do under earlier versions of iMovie - until I happened upon the solution described above. I'm using iMovie 10.1.4 but the way.
Have just exported my completed movie to file, and ALL the clips are there - so the method above does clear the clips from the 'my movie' pane without removing them from the active project.