Back worst runner at 'Aydock? (7)

I believe the answer is:

endorse

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'back' is the definition.
(I know that endorse is a type of warrant)

I can't explain the remainder of the clue.

(Other definitions for endorse that I've seen before include "Advocate" , "Put one's name to, approve" , "Give one's sanction to" , "Give backing to" , "Docket" .)

I've seen this clue in The Telegraph.

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