Cutting off part of a video can mean two different things: trimming the duration (removing frames from the start or end) or cropping the frame (removing edges from the visible area).
For trimming by duration, use the iOS’s native Photos app: open the video in Photos, tap Edit, and drag the yellow handles at the start and end of the clip.
For cropping the frame – which Photos cannot do for video – use Movavi App: import the clip, tap the Crop tool, crop the unwanted edges, and export. The result saves as a new file to your camera roll with the original left intact.