Clen is one of those drugs you have to take something else with, and that something else is T3 (clen suppresses thyroid function).
T3 is not specific as to what it will have the body metabolise, so it will burn fat and muscle.
Burn muscle, you lose the best metabolic engine and the tissue that gives great shape to the body.
So then people take an anabolic/gear with clen/T3 to prevent muscle loss.
Either way, just clen or clen and T3, when you come off, your thyroid gland is NOT going to be making T4 (it gets converted to T3 in the peripheral tissue, T3 feeds back to the thyroid gland to say 'don't make more'), so your metabolism will be slower, even with good diet and cardio and weight training.
Without diet, training, cardio in place, IMHO, it is a recipe for disaster as far as a nice physique/body goes.