"Yesterday!" (4)
I believe the answer is:
asap
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">yesterday</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)