Rush order letters (4)
I believe the answer is:
asap
'rush order 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 one's earliest convenience" , "At the earliest opportunity (abbrev.)" , "Common acronym for quick-as-you-can" , "As soon as you can, initially" , "'As quickly as you can, initially (4)'" .)