Shrub, one found by river (4)
I believe the answer is:
acer
'shrub' is the definition.
(I've seen this before)
'one found by river' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'found by' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'ace'+'r'='ACER'
(Other definitions for acer that I've seen before include "Tree or shrub of the maple genus" , "Care about this tree" , "Plant" , "ornamental tree" , "Type of maple" .)