Ditch one hectare after another (2-2)
I believe the answer is:
ha-ha
'ditch' is the definition.
(type of ditch)
'one hectare after another' is the wordplay.
'one hectare' becomes 'ha' (abbreviation for hectare).
'after' is a charade indicator (letters next to each other).
'another' becomes 'ha' (I can't justify this - if you can you should believe this answer much more).
'ha'+'ha'='HA-HA'
(Other definitions for ha-ha that I've seen before include "Sunken barrier" , "that I find amusing" , "Sound of laughter" , "Representation of laughter" , "Low ditch with wall in it" .)