Exist as regions (5)
I believe the answer is:
areas
'regions' is the definition.
(I know this)
'exist as' is the wordplay.
'exist' becomes 'are' ('they exist' means 'they are').
'are'+'as'='AREAS'
(Other definitions for areas that I've seen before include "Spaces covered by two-dimensional surfaces" , "Localities" , "Places" , "Geographical regions" , "Zones" .)