It depends on how short you cut the feathers and how many of them. Remember, the feathers grow back in.
My experience is with parrots, not ducks but I expect it would apply. the idea of clipping one wing makes it harder for them to control the direction of their flight. I might consider some netting over their enclosure.
Thanks guys. Hmm may have to pinion in that case, since I've heard it's more effective and people haven't seen their birds fly more than 3 feet high after the procedure. I hate to make them literally "sitting ducks" but I can't afford them flying over the fence with Muscovy laws, and this is a huge area (1/2 acre) I don't think a top would be very economical.