Player adjusted clipping plane,

I'm making a squad-based tactics game, and I need the player to be able to able to adjust a clipping plane that's not in line with the main camera.

Tops of trees, and floors the player isn't on needs to not be rendered. They need a control to adjust the height of the clipping plane at will, and the angle of the main camera should have no effect on this.

Buildings are generated dynamically as a single object, so partial transparency isn't an option. It needs to be a horizontal adjustable clipping plane. Main camera angle will not be hard-set.

Thanks.,