Act One needs rewriting straight away (2,4)
I believe the answer is:
at once
'straight away' is the definition.
'at once' can be an answer for 'away' (thesaurus). I am not certain of the 'straight' bit.
'act one needs rewriting' is the wordplay.
'needs rewriting' indicates anagramming the letters (I've seen this in other clues).
'act'+'one'='actone'
'actone' with letters rearranged gives 'AT ONCE'.
Can you help me to learn more?
(Other definitions for at once that I've seen before include "simultaneously" , "Immediately, this instant" , "without hesitation" , "right away" , "This instant - no delay (2,4)" .)