Dieter’s concern, perhaps, consuming zero chocolate substitute (5)

I believe the answer is:

carob

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'chocolate substitute' is the definition.
(carob powder is a chocolate substitute)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for carob that I've seen before include "Evergreen tree with edible pods" , "Saint John's bread" , "Arabian tree with brownish-purple pods" , "St John's bread" , "Chocolate substitute pod" .)

I've seen this clue in The Times.

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