Drink ruined ace diet (4,3)
I believe the answer is:
iced tea
'drink' is the definition.
(I've seen this before)
'ruined ace diet' is the wordplay.
'ruined' indicates an anagram.
'ace'+'diet'='acediet'
'acediet' with letters rearranged gives 'ICED TEA'.
(Other definitions for iced tea that I've seen before include "Cool drink" , "Chilled drink" , "Cold summer drink" .)