I have heard it said that smaller chickens, like bantams and breeds a little closer to wild like Araucanas can fly much much better than heavy breeds.
My standard Cochins, speaking from their perch in the 10' high barn rafters, tell me that this is a total lie and they will fly wherever they darn well please, no matter what I do to their wings.
If they really want to fly over a fence, they will figure it out, clipped wings or no. If you don't want them to fly over the fence, your best option is to enclose their run completely.