As soon as yesterday (4)
I believe the answer is:
once
This clue is a double definition.
'as soon as' is the first definition.
(once can mean immediately after)
'yesterday' is the second definition.
(I've seen this before)
(Other definitions for once that I've seen before include "A single time" , "Singly" , "... upon a time for the storyteller" , "Formerly, at one time" , "Just as a single occurrence" .)