Highly debated topic. Some say that it has anti-catabolic effects and others insist that it does not distihguish between what is and isn't burned.
However, I wouldn't worry about muscle loss while using clen if your on cycle at all. Some say that it actually has some anabolic "properties" but there isn't much, if any, evidence to support this.
It will help you cut up though, as long as your diet and cardio are in check.