As soon as (4)
I believe the answer is:
once
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">as soon as</span>' is the definition.<br/ ><span class="explanation-format__explanation">(once can mean immediately after)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for once that I've seen before include "At some time past" , "One-time musical film and song" , "not repeatedly" , "In times past" , "One-time - one time" .)