Urgent letters (4)
I believe the answer is:
asap
'urgent letters' 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 "yesterday?" , "'At the earliest moment, initially (4)'" , "At the earliest opportunity (abbrev.)" , "At one's earliest convenience" , "As soon as you can, initially" .)