A loser anyone can back (3)

I believe the answer is:

dud

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'a loser' is the definition.
(I know that dud is a type of loser)

I cannot understand how the rest of the clue works.

(Other definitions for dud that I've seen before include "Broken" , "Ineffectual person" , "Faulty" , "(Explosive) failure" , "Palindrome for a flop" .)

I've seen this clue in the Evening Standard.

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