Dodgy diet - not a cure (8)
I believe the answer is:
antidote
'cure' is the definition.
(antidote is a kind of cure)
'dodgy diet not a' is the wordplay.
'dodgy' indicates an anagram.
'diet'+'not'+'a'='dietnota'
'dietnota' anagrammed gives 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "Tied to an awful counteracting agent" , "It neutralises poison" , "Counterpoison" , "Counter-agent" , "Cure for poison" .)