This iced tea's terrible, but it's too late to change (3,3,2,4)
I believe the answer is:
the die is cast
'it's too late to change' is the definition.
(if 'the die is cast', events are already settled)
'this iced tea's terrible' is the wordplay.
'terrible' is an anagram indicator.
'this'+'iced'+'teas'='thisicedteas'
'thisicedteas' anagrammed gives 'THE DIE IS CAST'.
'but' acts as a link.
(Other definitions for the die is cast that I've seen before include "There's no turning back now" , "This step is irrevocably taken" , "An irrevocable step is taken" , "Remark on crossing the Rubicon" .)