Immediately in agreement about opening of casino (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(thesaurus)
'in agreement about opening of casino' is the wordplay.
'in agreement' becomes 'atone' (I've seen this in other clues).
'about' is an insertion indicator.
'opening of' suggests taking the first letters.
The first letter of 'casino' is 'c'.
'atone' going around 'c' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "Instantly" , "on the spot" , "Octane (anag)" , "'Immediately (2,4)'" , "Right now, this instant" .)