Can make one act straightaway (2,4)
I believe the answer is:
at once
'straightaway' is the definition.
(both can mean immediately)
'can make one act' is the wordplay.
'can make' indicates anagramming the letters (I've seen this in other clues).
'one'+'act'='oneact'
'oneact' with letters rearranged gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "without pause" , "A no etc (anag)" , "Immediately, this instant" , "Right now, this instant" , "Instantly" .)