A failure as a keeper (5)

I believe the answer is:

loser

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'a failure' is the definition.
(someone who has failed)

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

(Other definitions for loser that I've seen before include "Generally inept" , "Opposite of winner" , "Unsuccessful type" , "A failure, also-ran" , "one plays without success" .)

I've seen this clue in the Evening Standard.

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