Cliptool.
Very easy, you use it in the 2d views.
you select the brush you want to clip, and in the 2D view, you drag lines over it and clips it, stright over.
You might wanna click the tool like, 3 times, else, one of the sides you clip over will dissapare.
I suggest, draw a clip line, and click the tool untill no side is red.