That depends on the weight of the bird. They can still fly pretty well if you clip both wings, you should probably clip just one wing, this way when they try to fly they will spiral and then land.
I have a a chicken that is part banty and part standard and she could fly up over 20 feet, I clipped her one wing so I wouldn't lose her, but she won't fly away anymore she'd rather stay with her buddies.
If they wanted to they will fly over a 4 foot fence easily, and six foot without a problem. Clipping only one wing will make it a little more difficult for them.