Back worst runner at 'Aydock? (7)
I believe the answer is:
endorse
'back' is the definition.
(I know that endorse is a type of warrant)
I can't explain the remainder of the clue.
Can you help me to learn more?
(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" .)