Here is a solution to the problem:
In the Finder -> Go -> Go to Folder,
enter /Users/Your user name//Movies/iMovie Library.imovielibrary/Name of Event/
(Name of Event is exactly as it appears in the Event Browser)
This takes you to the folder in iMovie Library for that event, and you will see the contents of that folder as several folders and perhaps a file. One of the folders is Original Media. Open that folder and you will see individual files of the individual clips contained in that Event. Delete the file of choice in the usual OS X way. iI will then appear in the Trash folder. Empty the trash and it is gone.
This solution was suggested by Shirt Pocket Support, which provides technical help for the backup app SuperDUper. I am indebted to them.