Expression started daft argument over there (4)

I believe the answer is:

word

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'expression' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for word that I've seen before include "Puzzle with solutions reading the same down as across" , "Term" , "Element of language" , "Promise; news" , "Brief statement" .)

I've seen this clue in The Irish Times.

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