But it is actually the bookies’ final offer (8,5)
I believe the answer is:
starting price
'bookies' final offer' is the definition.
(odds on a horse immediately prior to a race)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for starting price that I've seen before include "Odds on horse at beginning of race" , "Final odds as race begins" , "Key information for better" , "Racing tipster (anag.)" .)