Change of diet not a remedy (8)
I believe the answer is:
antidote
'remedy' is the definition.
(antidote is a kind of remedy)
'change of diet not a' is the wordplay.
'change of' is an anagram indicator.
'diet'+'not'+'a'='dietnota'
'dietnota' is an anagram of 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "Medicine countering a poison" , "Counter-agent" , "Counteracting medicine" , "Counter to a poison" , "It couneracts poison" .)