If this is how you want it:
A clear screen, no effects, just a "movie" live from the camera.
then.
I think, that there is an entity, info_camera_link, it can make you target which brush you want the camera to be connected with, so, like, if you make a brush, make it nodraw on all sides, the, target the "camera viewpoint entity" to one of the brush sides.
If you look in the entity menu, you will find the "camera viewpoint entity" easy, the name explanes it all.
I think you used this method to make a TV, I learned it over at snarkpit,
here's the tut URL, it might help you:
http://www.snarkpit.net/editing.php?page=tutorials&id=127&highlight=Monitor