welcome to the MPV forums :)
I'm not near FCP right now but there are a few ways to do this. Here's one way:
1. Select the clip/(s) in the Timeline -- shift-click to select multiple clips.
2. If you want to select an area which is not split as different clips, et the In and Out points around this area to be rendered.
3. Choose Sequence > Render Selection > ...Both / Video or Audio depending on what you wish to render.
Hope this helps