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French menu item missing key idiom (5)

Ross

I believe the answer is:

argot

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'idiom' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

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(Other definitions for argot that I've seen before include "Slang of a particular group, originally thieves" , "Slang, patois" , "Special jargon of a group" , "Ancient Greek coin" , "secret language" .)

I've seen this clue in The Independent.
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