If the clip came across from your iPhone through iCloud Photos, or you imported it into your library, Photos is the natural place to fix it.
- Double-click the video in Photos to open it.
- Click Edit in the toolbar.
- Click the Rotate button at the top of the window. Each click turns the video a quarter turn counterclockwise, so three clicks equal one clockwise turn.
- Click Done to keep the change.
Photos stores your edit next to the untouched original. To go back, open the video, click Edit and choose Revert to Original. With iCloud Photos switched on, the rotation also syncs to your iPhone and iPad.
For a file you can send or upload, select the video and choose File → Export → Export 1 Video. Skip Export Unmodified Original for this job, since it hands you the sideways version you started with.