Urgent acronym (4)
I believe the answer is:
asap
'urgent acronym' 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 "Notation for urgent, quick as you can (4)" , "As soon as you can, initially" , "At the earliest opportunity (abbrev.)" , "yesterday?" , "'At the earliest moment, initially (4)'" .)