Around river, retain new land (7)
I believe the answer is:
terrain
'land' is the definition.
(I've seen this before)
'around river retain new' is the wordplay.
'around' is an insertion indicator.
'river' becomes 'r'.
'new' indicates anagramming the letters.
'retain' anagrammed gives 'terain'.
'r' placed inside 'terain' is 'TERRAIN'.
(Other definitions for terrain that I've seen before include "Stretch of land with respect to its character" , "(Type of) ground" , "Area of ground with a particular characteristic" , "Tract of land with prevailing characteristic" , "Ground - rare tin (anag)" .)