"Quickly!" letters (4)
I believe the answer is:
asap
'quickly letters' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for asap that I've seen before include "At the earliest opportunity (abbrev.)" , "Notation for as quickly as you can" , "Notation meaning without delay, treat as urgent" , "At one's earliest convenience" , "As soon as possible (abbrev.)" .)