Oddball diet not a cure (8)
I believe the answer is:
antidote
'cure' is the definition.
(antidote is a kind of cure)
'oddball diet not a' is the wordplay.
'oddball' is an anagram indicator (I've seen this in other clues).
'diet'+'not'+'a'='dietnota'
'dietnota' is an anagram of 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "Counteracting medicine" , "It is used to counteract poison" , "simple remedy" , "Agent counteracting a poison" , "A remedy for poison (8)." .)