Call of Duty bo3: Clipping Method1

Auto model clipping in Radiant

With your map open in Black Radiant select the clip texture you need in the Textures list
Select the model you wish to clip
Go To Select > Make Geo > Generate Clip

You are done!

NOTE: With some models the clip generated might end up having too many faces and could lead to compile failing!

Thanks to Steve @MapperUnited.com for this! link(registration required)