Bad apple or sour grapes, e.g. (5)

I believe the answer is:

idiom

(Other definitions for idiom that I've seen before include "Language used by group, parlance" , "Local style of expression" , "Language used by people or group, vernacular" , "Way of speaking" , "Saying specific to a certain language" .)

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

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