a 10-second video clip. "seconds" is used as an adjective here, so it doesn't get an 's'. (We don't say : 2 reds chairs, or 5 browns dogs) So, five 10-second video clips. (5 clips of 10 seconds each) for example. a video clip two video clips a ten-second video clip two ten-second video clips

A 10 second video clip.