Concealed ditch in landscape gardening (2-2)
I believe the answer is:
ha-ha
'concealed ditch in landscape gardening' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ha-ha that I've seen before include "Ditch with wall in it - sounds hilarious" , "Sunken barrier" , "Sound of laughter" , "Laugh - ditch acting as fence" , "Funny type of ditch?" .)