Depends how fast you want to reach that aim - and 6-7% is very low.
If you don't want to use any substances, just train hard and diet very strict (I know, it's like a standard sentence but that's the biggest part of such a aim).
Ostarine in that case would be interesting because of the anti-catabolic/anabolic effect. But if there would be a restriction not to use any SARM or AAS there are some other opportunities.
First of all, a classic EC(A)Y-Stack would speed up the way to your goal. Also Yohimbine as a stand alone has enormous benefits and together with some Ephedrine there would be a very nice effect. Also green tea is very underrated. Try to drink some liters of green tea during the day - it will heat you up a bit and generating a bigger kcal-loss.
Despite of the strong effects of legal substances like caffeine or green tea there would also be GW-501516 as a PPAR. It's also a research chemical and I wouldn't exceed the usage of this more than twice a year.
GW in this case would have a strong anti-catabolic effect and would be probably the "best semi-legal" research chemical in that case. Main problem is that GW is - as said - a research chemical and there are no long term studies yet. I've run it serveral times (not for fat loss in first case but for the endurance component) and until now everything is fine. But that's not a garant for a possible turnover.
If using GW, also use the root Tribulus. It acts as one of the strongest COX-2 inhibitors (over 80%). It is shown that GW in already existing tumours can speed up the growth. So, don't use GW under any circumstances if you have or had cancer.
In the dosage which were tested on human beeings, they were none reported side effects liked with tumor growth. But better stay on the safer side and spend some money on Tribulus.
Another option would be the usage of a thyroid hormone like T3. I personally wouln't touch this and I don't have any experience so I can give you only theoretically knowledge. But in the worst case you will be reliant to thyroid hormone for your whole life.
What I strongly don't recommend is the abuse of Clenbuterol.
So as a conclusion, try to run caffeine and some yohimbine (and some ephedrine). In most cases this is enough and play with some powerful legal substances. GW would also be an usable opportunity but it's also a research chemical. But so far as I can say, in healthy humans GW in normal dosages shouldn't be a big problem (personal opinion).
I personally don't recommend the usage of T3/T3.5//T4 and Clenbuterol.