River passing through excellent piece of ground (4)
I believe the answer is:
acre
'piece of ground' is the definition.
(a piece of land)
'river passing through excellent' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'passing through' is an insertion indicator.
'excellent' becomes 'ace' (ace can informally mean very good).
'r' placed within 'ace' is 'ACRE'.
(Other definitions for acre that I've seen before include "Unit of area - Israeli port" , "stretch of land" , "Four thousand, eight hundred and forty square yards" , "Care about the piece of land" , "Site of battle" .)