Urgently (4)
I believe the answer is:
asap
'urgently' 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 one's earliest convenience" , "yesterday?" , "Notation meaning without delay, treat as urgent" , "'At the earliest moment, initially (4)'" , "Notation meaning with minimum delay" .)