Act One goes wrong immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(thesaurus)
'act one goes wrong' is the wordplay.
'goes wrong' is an anagram indicator.
'act'+'one'='actone'
'actone' anagrammed gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "A no etc (anag)" , "Immediately, this instant" , "Right now, this instant" , "'Immediately (2,4)'" , "This instant, hop to it" .)