A diet not working as remedy (8)
I believe the answer is:
antidote
'remedy' is the definition.
(antidote is a kind of remedy)
'a diet not working' is the wordplay.
'working' indicates anagramming the letters.
'diet'+'not'='dietnot'
'dietnot' with letters rearranged gives 'ntidote'.
'a'+'ntidote'='ANTIDOTE'
'as' acts as a link.
(Other definitions for antidote that I've seen before include "It couneracts poison" , "Donate it to anti-poison agent" , "It counteracts poison" , "Counteractive medicine" , "Cure for poison" .)