Initials indicating urgency (4)
I believe the answer is:
asap
'initials indicating urgency' 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 "'As quickly as you can, initially (4)'" , "'At the earliest moment, initially (4)'" , "Common acronym for quick-as-you-can" , "As soon as you can, initially" , "Notation for urgent, quick as you can (4)" .)