Plain fine diet perhaps (8)
I believe the answer is:
definite
'plain' is the definition.
(thesaurus)
'fine diet perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'fine'+'diet'='finediet'
'finediet' is an anagram of 'DEFINITE'.
(Other definitions for definite that I've seen before include "Clearly marked out" , "Unmistakable" , "Explicit, positive" , "Certain or explicit" , "Sure, certain" .)