I too have had the experience that clipping one wing does nothing to impede a chicken's ability to get over the fence. It works for budgies, but not for chickens. I hypothesize that this is because chickens don't so much fly as they jump with flapping.
EDT: @centrarchid actually did a study using his birds and found that clipping one wing is far less effective than clipping both is.
EDT: @centrarchid actually did a study using his birds and found that clipping one wing is far less effective than clipping both is.