That's an FOF. Here's how to make it:
You make a sector outside of the level. Then, you set its ceiling to the height you want the top of the FOF to be. Then, you set its floor to the height you want the bottom of the FOF to be. Then, you attach the linedef special you want to one side of the sector. Then, you attach the sidedef you want for the FOF to the same side. Then, you tag the linedef to the sector inside the level that you want the FOF to be over.
If you didn't understand all that, check the Wiki. That's what you should have done in the first place.