Clen is very much alike Ephedrine, your body will become accustomed to the dosage in a short period of time, thus which is why it is better to do an 2 week on, and 2 week off of clen.
Adding it to week 11 of the 10 weeker would be benefitcal because you will still be consuming extra calories and while coming off the AAS will help you minimize any fat gain along with other good advantages of clen.
What kind of clen are you planning to use?