I have never used Clen, but I have I had very good results using a couple of ephedra/caffeine/yohimbine and norephedrine/caffeine/yohimbine products stacked with a couple of tiratricol/triac products.
The thermogenics do two things for me:
1) counterbalance the sluggishness I feel from the triac alone, and
2) help preserve lean body weight and strength, which is what you are concerned about.
On my first cycle using triac alone, I lost about 10 pounds in 20 days, but I felt tired, and my lifts suffered noticeably. After that, I've always used a thermogenic product in conjunction with the triac.
I tend to lose from 5 - 8 pounds of bodyweight over a 14 day cycle using a decent (and sometimes less-than-decent) nutritional and training program, and I would guestimate that 4 - 7 pounds of that is "non-muscle" weight loss.