Boundary marker set in ditch? Very funny! (2-2)
I believe the answer is:
ha-ha
This clue is a double definition.
'boundary marker set in ditch?' is the first definition.
(type of garden ditch)
'very funny' is the second definition.
(Other definitions for ha-ha that I've seen before include "Funny type of ditch?" , "Sunken barrier" , "Ditch with wall in it - sounds hilarious" , "Concealed ditch in landscape gardening" , "Laugh - ditch acting as fence" .)