You can clip by just pinching the very newest set of leaves off the top. This will make the plant bush out more and promote thicker foilage. It will not hurt the plant in the least. I know it's hard to do at first as it feel like your hurting your baby, but it will be okay. If your grow room isn't the largest it will be nessasary.

Also, you could look up how to FIM you plants. This way you sometimes get more than three growing points as compared to a regular clipping. This method isn't always consistant but it can't hurt to try. Hope this helps - good luck!
thnx for that mate! thats very helpfull....