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 quite understand how this works, but
'last' could be 'end' (I've seen this in another clue) and 'end' is found within the answer.
'runner' could be 'r' (abbreviation) and 'r' is present in the answer.
The remaining letters 'ose' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
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)" .)