Unknown: a single area (4)
I believe the answer is:
zone
'area' is the definition.
(zone is a kind of area)
'unknown a single' is the wordplay.
'unknown' becomes 'z' (algebra).
'a single' becomes 'one' ('one' can be a synonym of 'single').
'z'+'one'='ZONE'
(Other definitions for zone that I've seen before include "District, area" , "Bitter, pungent" , "One of five belts round the earth" , "London Underground region" , "Area; belt" .)