A French cookery writer reportedly never surpassed (8)
I believe the answer is:
unbeaten
'never surpassed' is the definition.
(I've seen this before)
'a french cookery writer reportedly' is the wordplay.
'a french' becomes 'un' ('a' in French).
'cookery writer' becomes 'beeton' (Victorian writer Mrs Beeton).
'reportedly' shows a homophone (sound like).
'beeton' is a homophone of 'beaten'.
'un'+'beaten'='UNBEATEN'
(Other definitions for unbeaten that I've seen before include "never surpassed" , "there's nothing better" , "Not yet defeated" , "not having suffered defeat" , "all-time" .)