Split/edit part in FreeCAD?
When you want to change an STL file in FreeCAD then this is how I do it. I open the STL file in FreeCAD and select the option in the Part/Component sub-menu: Make a form from mesh (hope that I translated it well. I use the Dutch version). It can take quite some time but when it is finished you have an editable object. Just remove the STL object and do what you want.

When you finished then you can perform all actions like if you created it from scratch with FreeCAD.

I am a beginner in 3D design so there might be 'smarter' solutions. When I want 1 special part then I create as many cubes as needed and perform a MINUS action on the solid and a cube. After a while I end up with only the part I need.