Appear unexpectedly (3,2)
I believe the answer is:
'appear unexpectedly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pop up that I've seen before include "Appear suddenly or unexpectedly" , "three-dimensional picture book" , "which may interrupt computer user" , "Appear (like toast?)" , "book for youngster" .)