Forthwith, at once (9)
I believe the answer is:
instantly
I believe this is a double definition.
'forthwith' is the first definition.
(I know that forthwith can be written as instantly)
'at once' is the second definition.
(I've seen this before)
(Other definitions for instantly that I've seen before include "Immediately, forthwith" , "With no delay, this very second" , "Without delay or hesitation" .)