Confirm return of top-class racing with ideal setting (6)
I believe the answer is:
verify
'confirm' is the definition.
(verifying is a kind of confirming)
I don't understand how the rest of the clue works.
(Other definitions for verify that I've seen before include "Check the truth of" , "Check (facts)" , "Establish truth or accuracy of" , "Ascertain truth or accuracy" , "Demonstrate" .)