River found in one's estate (5)
I believe the answer is:
acres
'estate' is the definition.
(synonyms)
'river found in one's' is the wordplay.
'river' becomes 'r' (abbreviation).
'found in' is an insertion indicator.
'one's' becomes 'ace's' (ace has a value of one in some card games).
'r' going within 'aces' is 'ACRES'.
(Other definitions for acres that I've seen before include "may burden landed folk" , "lots" , "Cares about the tracts of land" , "Scare (anag)" , "large area of land" .)