If you sync video, you cannot delete it straight from iMovie.
You need to use iTunes and un-sync it.
If you shot video directly with device within iMovie Project, you can manage local media by doing this:
In iMovie iOS Project
In Project Timeline DELETE CLIPS that you no longer need
Open Video Bin
Tap "All" icon
Tap "Manage local Media"
Tap Done
In the Video Bin you will see two icons to the right of all clips you shot locally on device and those you imported via iMovie Sharing function in iTunes.
Red icon is a trash can. Tap it. Clip is now gone.
If you attempt to delete in Video Bin before removing your clips in Timeline, you will get a warning saying that you are about to delete clips that are in USE