Copperpostie may well be right. But a Haha has no wall or fence. It is just a straight sided dividing ditch. I have one, which is at least 100 years old.
Apart from that, surely it is slightly against accepted convention that a key word in the clue should be a key word in the solution. KM is trying to make things harder, fine, but perhaps this should not be achieved with poor clue phraseology.