Atkins diet no-no (4)

I think it could be either:

Which length description are you looking for?:

carb

'atkins diet no-no' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for carb that I've seen before include "Carburettor (abbrev.)" , "Fuel" , "Substance such as sugar or starch in foods" .)

I've seen this clue in the Universal.

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