River in super area (4)
I believe the answer is:
acre
'area' is the definition.
(I've seen this before)
'river in super' is the wordplay.
'river' becomes 'r'.
'in' means one lot of letters goes inside another.
'super' becomes 'ace' ('ace' can be a synonym of 'super').
'r' put within 'ace' is 'ACRE'.
(Other definitions for acre that I've seen before include "Care about the piece of land" , "Site of battle" , "region of ground" , "Imperial unit of area" , "Race (anag.)" .)