Wall or fence set in a ditch so as not to interrupt the landscape (2-2)
(Other definitions for ha-ha that I've seen before include "Loud laugh" , "Sunk fence" , "Feature of posh garden" , "Hidden ditch" , "that I find amusing" .)
(Other definitions for haha that I've seen before include "Ditch" , "Laugh" , "The sound of laughter" .)