Clenbuterol is a hardcore selective b2 adrenalinic receptor agonist, meaning it will, among other things, directly stimulate fat cells to release fat and increase your body temperature through increased thermogenesis, which means burning more calories than normal even without activity.
It is known for fucking with your heart, (not right away, you might realize damage has been done years after discontinuing use) the more you use it the higher the chances ofcourse.
This is due to how strongly it activates the b2 receptors and also it's half life of over 30 hours.
It will rapidly downregulate said receptors, for it to be effective for more than 2 weeks you need to combine it with a drug that will upregulate the b2 receptors, ketotifen being the the best choice as it's the only one widely used with success.
I recommend that nobody uses clenbuterol, it's dangers far outweigh the rewards imho.
A very effective alternative is albuterol (the oral version, not the inhalers), it's got similar effects but a more reasonable half life of 5 hours, meaning that you can take it in the early afternoon and be almost completely off by the time you go sleep.
It won't burn fat as fast, but it's much safer.
To use a metaphor, using clean instead of albuterol to lose fat is like using meth instead of amphetamine salts for ADD.