Originally posted by Rickson
Clen does not induce muscle growth so you will only get harder by weight training and dieting. You have only been at it for 13 days so you should wait a little longer before being disappointed.
Clen doesn't induce muscle growth but it has an anti-catabolic effect and therefore it inhibits the breakdown of muscle tissue. It's also a thermogenic stimulant so it speeds up your metabolism and burns off fat as heat.

In theory you should be able to train harder and improve muscle mass and definition while burning off fat.

I suggest you stick to a regular high weight/low rep (3 sets of 8) routine to improve your muscle definition and size but make sure you do (at least) a couple of cardio sessions each week. Eat plenty of protein and reduce the intake of fatty and processed foods.