Suddenly appear (5)
(Another definition for popup that I've seen is " What 11 [TOAST] might do".)
'suddenly appear' is the definition.
(I've seen this before)
This is all the clue.
(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" .)