Back last runner at 'aydock? (7)
I believe the answer is:
endorse
'back' is the definition.
(back can mean to support or endorse)
'last runner at 'aydock?' is the wordplay.
I cannot really understand how this works, but
'last' could be 'end' (I've seen this in another clue) and 'end' is located in the answer.
'runner' could be 'r' (abbreviation) and 'r' is located in the answer.
The remaining letters 'ose' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for endorse that I've seen before include "Champion" , "Pass" , "Approve - support" , "Sanction -- red nose (anag)" , "Put penalty points on (a driving licence)" .)