Two horses going over a ditch (2-2)
I believe the answer is:
ha-ha
'ditch' is the definition.
(I know that ha-ha is a type of ditch)
'two horses going over a' is the wordplay.
'two' shows that letters should be duplicated.
'horses' becomes 'h' (this might be a standard abbreviation I've not previously seen).
'going over' is a charade indicator (letters next to each other).
'h'+'a'='ha'
'ha' doubled is 'HA-HA'.
Can you help me to learn more?
(Other definitions for ha-ha that I've seen before include "Ditch with wall in it - sounds hilarious" , "Sunk ditch - laugh!" , "that I find amusing" , "Sunk fence" , "Ditch with wall in it - sounds merry" .)