Fancy diet not a cure (8)
I believe the answer is:
antidote
'cure' is the definition.
(I know that antidote is a type of cure)
'fancy diet not a' is the wordplay.
'fancy' indicates an anagram.
'diet'+'not'+'a'='dietnota'
'dietnota' anagrammed gives 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "Medicine countering a poison" , "Agent counteracting a poison" , "Counterpoison" , "It counteracts poison" , "Counteracting remedy" .)