To clip a video, open the video you want to clip and hover over the gear icon on the top right corner of the video. Then, select 'Clip.' You can only clip a video if you own the video or if you made a duplicate copy of a video that someone shared with you at the High permission level.
After you select 'Clip,' you will then be able to drag the markers to only include the part of the video you would like to keep. Click the green check mark when you are done. You will then be able to find the clipped video on your Videos & Files page.