Variable quantity in region (4)
I believe the answer is:
zone
'region' is the definition.
(I know that zone is a type of geographic area)
'variable quantity' is the wordplay.
'variable' becomes 'z' (algebra).
'quantity' becomes 'one' (one is a kind of quantity).
'z'+'one'='ZONE'
'in' is the link.
(Other definitions for zone that I've seen before include "Particular area or sector" , "Marked area" , "Area, division" , "Section" , "Area; belt" .)