Atkins diet no-no (4)
'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" .)
(Other definitions for sugar that I've seen before include "--- Ray Robinson, boxer" , "Cane or beet product" , "Mild oath (granulated?)" , "Sweet ingredient" , "Sucrose" .)