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