Suddenly appear (5)

I think it could be either:

Which length description are you looking for?:

3,2

pop up

'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" .)

I've seen this clue in The Times.

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