Clean eating isn't perfect eating. It is trying to eating healthy and omit certain foods most of the time. At least that is my definition. Cheats are not such a bad thing if they are kept in moderation. Tosca Reno suggests once a week, some people do it once a day (which can ruin your calorie count). I personally think it depends on your goals. If you are actively trying to loose weight then once a week or less is a good target. If you are not, then you may be more cheatful.
Cheating is when you eat something out of your diet (plan) It can be beneficial and harmful. It is beneficial when it is planned and the person savors every single bite and returns to their plan without further cheats. There is a theory out there that says that a calorie bump is good for the body so that it maintains its ability to deal with excess calories. Whether this is true or not I feel like a cheat is more beneficial, however, mentally. It makes a person not see the diet as "the enemy" that keeps them from all that is good. A healthy cheat gives the person the feeling that their diet is part of them and is not a temporary thing.
A cheat can be harmful if it turns into a trigger. A trigger begins a binge. Creamy bar chocolate is a trigger for me so at this time I am avoiding it all together but I cheat on other things. My eating goes much better that way. Also a cheat can be harmful since in my experience sodium and sugar tend to kill the taste of healthier foods. Making my next healthy meal less appealing.
Some people have cheat days, cheat evenings, cheat meals or cheat weekends. Again I think it depends on a person's goals.