Back final runner at 'untingdon? (7)
I believe the answer is:
endorse
'back' is the definition.
(back can mean to support or endorse)
'final runner at 'untingdon?' is the wordplay.
I cannot really see how this works, but
'final' could be 'end' (synonyms) and 'end' is located in the answer.
'runner' could be 'r' (abbreviation) and 'r' is found 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 "Put penalty points on (a driving licence)" , "No reeds (anag)" , "Champion" , "Say you approve" , "Express approval" .)