If you are using the adjustment brush to make local image enhancements, you can hover over the pin to see the mask that you have drawn, but it shows for just a moment. To keep it on as you brush, type the letter O, for Overlay. To turn the overlay off, type O again. If instead you use the adjustment brush in Camera Raw, simply check the Show Mask check box to see the mask. (Why isn’t this check box in Lightroom?)
The clarity slider was introduced in Lightroom 1.1, and is also now in Camera Raw. A positive clarity value punches up an image, makes it look a little more three dimensional, by enhancing contrast along edges. The changes are concentrated in the midtones, and do little to highlights and shadows. Here is an example, a portion of an image with Clarity set to 0, and then set to 60.