Is it no good to eat? (4)

I believe the answer is:

duff

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'it no good to eat?' is the definition.
The definition suggests a singular noun which matches the answer.

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

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(Other definitions for duff that I've seen before include "False; steamed pudding" , "Pudding - of poor quality - Homer Simpson's favourite beer" , "Faulty (slang); a pudding" , "Bad" , "Worthless; pudding" .)

I've seen this clue in the Evening Standard.

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