A trailer and a short clip are two different things.
If you want to make a trailer, which means a short promotional clip that is made up of a montage of scenes from a movie, then you can certainly do that in Shotcut.
If you just want to pull a specific short clip in Shotcut from a larger clip without re-encoding then what you need to do is throw your video in the timeline, split the video up to the part that you want, select that clip, go to the Properties tab and pick Extract Sub-clip:
