How To Delete A Large Video File From iOS Device Which Does Not Show Up in Photos?
In the Photos app, select the Albums view and you'll see a Recently Deleted album there. Deleting the video from there will free the space for you. If you're not hurting for space, you can leave them and they'll be cleared from the device after 30 days. Each photo/video in the album is shown with the remaining countdown before the media is permanently removed by the OS.