Weak enough to fail outright (5)

I believe the answer is:

frail

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'weak' is the definition.
('frail' can be similar in meaning to 'weak')

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

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(Other definitions for frail that I've seen before include "American woman" , "Basket made of rushes" , "Easily destroyed" , "Feeble - basket" , "Struggling to get about" .)

I've seen this clue in the Evening Standard.

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