Illegal position of feet at dinner? (5-3-5)

I believe the answer is:

under-the-table

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'illegal' is the definition.
(thesaurus)

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

(Other definitions for under-the-table that I've seen before include "Hush-hush" , "Not above board" , "Illicit; hopelessly drunk" , "Transacted in secret -- sozzled" .)

I've seen this clue in The Times.

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