Rush order (4)
I believe the answer is:
asap
'rush order' 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 "Notation for as quickly as you can" , "Notation for urgent, quick as you can (4)" , "Notation or acronym meaning without avoidable delay" , "As soon as possible (abbrev.)" , "At the earliest opportunity (abbrev.)" .)