A behind's endless expanse (4)
I believe the answer is:
area
'expanse' is the definition.
('area' can be a synonym of 'expanse')
'a behind's endless' is the wordplay.
'behind' becomes 'rear' ('rear' can be a synonym of 'behind').
'endless' means to remove the last letter.
'rear' with its final letter removed is 'rea'.
'a'+'rea'='AREA'
(Other definitions for area that I've seen before include "Territory" , "field of study" , "Space occupied by something" , "Region, space" , "District; locale" .)