"Up in arms" or "break a leg" (5)

I believe the answer is:

idiom

(Other definitions for idiom that I've seen before include "Characteristic style, vocabulary" , "Characteristic language of people or group" , "Saying specific to a certain language" , "Colloquial saying" , "Phrase with a meaning that is not literal" .)

I've seen this clue in The New York Times.

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