He wants a clip brush that blocks players but lets npc pass. The clip texture blocks both.
I'm pretty sure this can't be done in just any situation. The only thing I can think of is to have two 1-unit-thick func_wall_toggles, both with the same name and maybe 64 units apart, that will act as your clip brushes. Put the player on one side and the npc on the other, and put a trigger half way between the two walls that targets the togglable walls and that can only be triggered by NPCs. Now when the NPC passes through the trigger the "clip" brush will "shift" behind the NPC, allowing it to come to you, and vice versa. Of course, two problems could arise: the player could get stuck when one wall is triggered, or if the player stands right next to one wall and they are triggered then the player will be able to pass into no man's land.