The healing brush is also good for this.
Select it from the toolbar (icon looks like an elastoplast). On the tool options bar at the top left of the screen, click the arrow next to the 'brush' settings. This gives you a dialog to adjust your brush size. Also make sure that the hardness of the brush is set to 0%.
Then position the cursor over a good area of skin, hold the ALT key down and click with the mouse (This samples the good skin area), then release the ALT key and click on the area of cellulite. After every couple of clicks press the ALT key again and re-sample a good area of skin.
The clone stamp is operated in exactly the same way. Try a few methods and see what works best.