Trench dug, yah boo sucks! (2,2)

I believe the answer is:

ha ha

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'trench' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for ha ha that I've seen before include "laughter" , "thing to keep animals out!" , "A wall" , "Boundary marker" , "Bound" .)

I've seen this clue in The Guardian.

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