I don't know about clen and yohimbe, but I wouldn't mix ECA and Yohimbe. I did it once, and I felt like total crap. You don't even want to workout, and insomnia is severe. You can try it, but I don't think you would like it. But topical yohimbe combined with ECA worked very well for me, with no negative side effects.
Clen should go well with yohimbe if you can take the sides. I have read that yohimbine HCL is an alpha2 receptor agonist in adipose tissue so I imagine there would be some synergy between clen and yohimbe.