Certain diet fine in the cooking (8)
I believe the answer is:
definite
'certain' is the definition.
(similar in meaning)
'diet fine in the cooking' is the wordplay.
'in the cooking' indicates an anagram (letters must be cooked or processed).
'diet'+'fine'='dietfine'
'dietfine' with letters rearranged gives 'DEFINITE'.
(Other definitions for definite that I've seen before include "Explicit, positive" , "Certain; clearly stated" , "Certain, decisive" , "The is the article" , "Clearly marked out" .)