A new diet not a remedy (8)
I believe the answer is:
antidote
'remedy' is the definition.
(I know that antidote is a type of remedy)
'a new diet not a' is the wordplay.
'a new' indicates an anagram.
'diet'+'not'+'a'='dietnota'
'dietnota' with letters rearranged gives 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "simple remedy" , "Counterpoison" , "It counteracts poison" , "Medicine to counteract poison" , "Counteracting medicine" .)