Suddenly appear (5)
(Another definition for popup that I've seen is " What 11 [TOAST] might do".)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">suddenly appear</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for pop up that I've seen before include "Appear suddenly or unexpectedly" , "Appear (like toast?)" , "three-dimensional picture book" , "which may interrupt computer user" , "book for youngster" .)