The clip tool works by selecting a brush, then drawing a line over it, the portion that is subsequently highlighted in white will be the result of the clip. You can choose to clip the brush on either sides of the line or cut the block into 2 separate pieces by clicking the clip tool icon in the sidebar after you've drawn the line in the grid. If you select multiple brushes the clip tool will clip all of the brushes along the line (making it suitable for making bsp/vis-friendly stairs and the like). If the clip tool doesn't seem to be waorking (you can't make a line), just hit Enter.
For making hallways and stuff, I really suggest making it out of several brushes rather than the clip tool. Clipping is really only useful for making more complex brushes (e.g hexagons, octagons, etc) or making pattern designs on walls, and even then for the latter I usually use Shift-Drag most of the time, especially if the brushes I'm making patterns on are grouped together because of brush entities and the like.