clip brushes for skybox

[freekill]

i was reading,
and read the the skybox around radar is clip brused
ive been looking for more detail info about them and havent seen anything to help me
or to really tell me what they r or how to make them
like in radar the bridge and beyond i can see like forever how is that done
(the bridge beside the bunker with barbed wire on it and a big pit under it)