French writer, unexpected rival to Racine, ultimately (8)
I believe the answer is:
voltaire
'french writer' is the definition.
(I know that Voltaire is French)
'unexpected rival to racine ultimately' is the wordplay.
'unexpected' is an anagram indicator.
'ultimately' suggests the final letters.
The final letter of 'racine' is 'e'.
'rival'+'to'='rivalto'
'rivalto' with letters rearranged gives 'voltair'.
'voltair'+'e'='VOLTAIRE'
(Other definitions for voltaire that I've seen before include "French satirist" , "French writer/philosopher" , "French writer (Candide)" , "French author of Candide, d. 1778" , "polemical writer" .)