Final Cut Pro X: Trim clips
Trim controls allow you to independently trim each of the image edges, creating a window look. This effect, often used along with the Transform effect, is commonly used on a clip placed over a primary storyline clip, which becomes the background of the trimmed clip.
Select a clip in the timeline.
To access the Trim controls, do one of the following:
Click the pop-up menu in the lower-left corner of the viewer and choose Crop (or press Shift-C).
Control-click in the viewer and choose Crop.
Click the Trim button at the bottom of the viewer.
Adjust the effect using the onscreen controls:
Blue handles at each corner: Drag these to adjust the position of two trim window sides at the same time.
Blue handles in the middle of each side: Drag these to independently adjust the position of each side.
Anywhere inside the window: Drag anywhere inside the window to adjust its position.
To individually adjust each edge using the Crop area of the Video inspector, use the Trim controls.
When you’re finished adjusting the effect, click Done.
You can animate the effect and have it appear over a background. For details on working with built-in effects, see Work with built-in effects.