"Yesterday!" (4)
I believe the answer is:
asap
'yesterday' 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 "Notation for as quickly as you can" , "Notation-acronym meaning without delay" , "As soon as possible (abbrev.)" , "yesterday?" , "Common acronym for quick-as-you-can" .)