Fold up show in area (10)

I believe the answer is:

concertina

'fold up' is the definition.
(to concertina is to fold up like a concertina)

'show in area' is the wordplay.
'show' becomes 'concert' (concert is a kind of show).
'area' becomes 'a' (maths abbreviation).
'concert'+'in'+'a'='CONCERTINA'

(Other definitions for concertina that I've seen before include "Musical instrument with bellows" , "Squeeze box" , "Collapse" , "gig belonging to American" , "fold" .)

I've seen this clue in The Times.

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