Hi,
It could be caused by a corrupt preference, or a glitch confined to one project, or perhaps not enough free space. You can try a few diagnostic procedures to see if you can isolate or cure the problem.
First check to see if you have sufficient free space on your drive for iMovie to function -- at least 25 MB for a movie that size.
Update to current iMovie 10 and Mojave if your computer specs allow.
If you are O.K. with the free space, and assuming you are using iMovie 10, try deleting preferences by restarting your computer and opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can import a clip into a project without crashes.
If still no luck, try booting up in Safe Mode (see Help menu instructions) and, after everything loads, immediately shut down and reboot in normal mode.
If still no luck, try importing into a newly created project.
Try opening iMovie in a new user account, such as the Guest account on your Mac.
All else failing, try a re-download of iMovie.
-- Rich