It's no good backing one, either! (3)

I believe the answer is:

dud

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'it's no' is the definition.
(thesaurus)

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

(Other definitions for dud that I've seen before include "Valueless" , "Not functioning" , "Explosive not detonating" , "A thing failing to work" , "Useless (informal)" .)

I've seen this clue in the Evening Standard.

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