One act play without delay (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this before)
'one act play' is the wordplay.
'play' indicates an anagram.
'one'+'act'='oneact'
'oneact' with letters rearranged gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "on the spot" , "This instant - no delay (2,4)" , "Immediately, this instant" , "This instant, without delay" , "This instant, hop to it" .)