Prove a race isn't fixed (9)
I believe the answer is:
ascertain
'prove' is the definition.
(thesaurus)
'a race isn't fixed' is the wordplay.
'fixed' indicates an anagram.
'race'+'isnt'='raceisnt'
'raceisnt' with letters rearranged gives 'scertain'.
'a'+'scertain'='ASCERTAIN'
(Other definitions for ascertain that I've seen before include "Find out for sure, determine" , "Prove" , "Find out - make sure" , "settle" , "Verify" .)