Tree, one by river (4)
I believe the answer is:
acer
'tree' is the definition.
(acer is a type of tree)
'one by river' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'ace'+'r'='ACER'
(Other definitions for acer that I've seen before include "Type of maple" , "Tree with winged fruit" , "ornamental tree" , "Tree or shrub of the maple genus" , "Care about the maple?" .)