Clipping is when you lose detail either by over-exposure, underexposure, or over-saturation.
Also called "crushed blacks," clipping is generally the first mistake people make when learning how to grade an image, usually from trying to achieve the highest contrast possible (it's okay, I was guilty of this at one point as well). The truth is that high contrast isn't always best, or more film-like. In fact, if you have ever worked with film, you will see that there is more detail in the blacks than digital footage, so if anything you should be more concerned with detail in the blacks.
Try using a vectorscope to make sure you aren't clipping, then lower the output level of the blacks just a tad if you want to mimic the look of film-stock.