Area surrounding river is outstanding (4)
I believe the answer is:
acre
'area' is the definition.
(I've seen this before)
'surrounding river is outstanding' is the wordplay.
'surrounding' is an insertion indicator.
'river' becomes 'r'.
'is outstanding' becomes 'ace' (I've seen this before**).
'r' inserted inside 'ace' is 'ACRE'.
(Other definitions for acre that I've seen before include "marine location in Israel" , "Persistent tendency" , "stretch of land" , "Levant sea port" , "4840 sq yds" .)