It happens to us all. Highlights get clipped. Shadows get blocked. What is a photographer to do? I'm glad you asked!
Tucked away in the corners of Lightroom's Develop module histogram are the clipping indicators. These little triangles can be of great assistance to you when you are adjusting your image and trying to pull details out of highlights and shadows. There is a clipping indicator for the highlights and one for the shadows.
Using the histogram you can see when clipping is happening. When you see spikes at either end of the histogram you can be sure you are losing some information in your image. But it can still be difficult at times to see all of the areas in the image that are blown out or blocked. The clipping indicators can help you target which areas are being clipped.
One way to use the clipping indicators is to hover for cursor over them. When you hover over the highlight clipping indicator all the blown out highlights will be colored red.
Hover over the shadow clipping indicator and all of the blocked shadow areas will turn blue.
While this can be useful it is difficult to adjust and hover adjust and hover. Luckily, you can click on either (or both) of the clipping indicators and keep the red and blue masks active. For the keyboard shortcut aficionados, press the J key to toggle both indicators on and off.
Now, with the clipping masks active, as you make adjustments you can watch the masked areas shrink. As they do you have immediate feedback that you are recovering highlight and/or shadow details!
Neat huh? But wait, there's more. While you are making adjustments pay attention to the clipping indicators. The Lightroom histogram shows you channel and luminosity values. It is possible to have one channel still clipping. If that happens, the clipping indicator will change it's color to match the channel that is being clipped. In this example, the highlight clipping indicator shows that the red channel is still being clipped. That let's me know I still have a little work to do.
Lastly, there are options available in the Develop module histogram. Simply right click on the histogram and the options menu will appear.
If you don't like the mouse over behavior simply uncheck the Show Clipping When Mouse is Over Indicator and the clipping masks will no longer appear when you hover over the indicator. You could also turn off the clipping indicators altogether (although I can't imagine why you would want to do that!)
This is another example of the richness of Lightroom's tools and interface. There are tons of great things tucked away in all of the modules. So explore Lightroom and remember Rule 5!