River discovered in private estate (4)
I believe the answer is:
tees
'river' is the definition.
(River Tees)
'discovered in private estate' is the wordplay.
'discovered' indicates a hidden word (remove the letters covering the answer in the clue).
'TEES' is hidden in the letters of 'in private estate'.
(Other definitions for tees that I've seen before include "Sides of Trent" , "Golfer's supports; river" , "Golfer's equipment" , "Golf ball props" , "Sport items" .)