"Pronto!" letters (4)
I believe the answer is:
asap
'pronto letters' 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 "At the earliest opportunity (abbrev.)" , "Notation or acronym meaning without avoidable delay" , "'At the earliest moment, initially (4)'" , "As soon as you can, initially" , "Initials notation meaning with all possible speed" .)