"Pronto!” in a memo (4)
I believe the answer is:
asap
'pronto in a memo' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for asap that I've seen before include "Notation-acronym meaning without delay" , "Initials notation meaning with all possible speed" , "yesterday?" , "Notation for urgent, quick as you can (4)" , "At one's earliest convenience" .)