Pudding not very good (4)

I believe the answer is:

duff

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'pudding' is the definition.
(I know that Duff is a type of pudding)

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

Can you help me to learn more?

(Other definitions for duff that I've seen before include "Heavy boiled or steamed pudding" , "Pudding - of poor quality - Homer Simpson's favourite beer" , "Dud - pud" , "Steamed pudding - 2" , "Bad" .)

I've seen this clue in The Times.

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