Walled ditch has repeatedly contracted

I believe the answer is:

haha

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'walled ditch' is the definition.
'ha ha' can be an answer for 'ditch' (I've seen this before). I'm not sure about the 'walled' bit.

'has repeatedly contracted' is the wordplay.
'has repeatedly' can mean two replacements for 'has'.
'has' becomes 'ha' (I can't explain this - if you can you should give a lot more credence to this answer).
'contracted' means to remove the last letter (to contract is to shorten).
'has' with its last letter taken off is 'ha'.
'ha'+'ha'='HAHA'

Can you help me to learn more?

(Other definitions for haha that I've seen before include "The sound of laughter" , "Laugh" , "Ditch" .)

I've seen this clue in The Daily Mail.

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