From underside, cored shallot? That's forbidden ... (2-2)

I believe the answer is:

no-no

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If this is the answer I cannot explain how the clue works.

(Other definitions for no-no that I've seen before include "Something that must not be done (informal)" , "Something that would be totally unacceptable" , "Non-starter" , "Impossibility" , "turn-off for some?" .)

I've seen this clue in The Telegraph.

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