Laugh; ditch (2-2)
I believe the answer is:
ha-ha
I believe this clue is a double definition.
'laugh' is the first definition.
(I've seen this before)
'ditch' is the second definition.
(I've seen this before)
(Other definitions for ha-ha that I've seen before include "Sunk ditch - laugh!" , "Ditch with a wall in it - that's a laugh" , "that I find amusing" , "Ditch with wall in it - sounds funny" , "Laugh - ditch acting as fence" .)