Bear close on river (6)
I believe the answer is:
endure
'bear' is the definition.
('endure' can be a synonym of 'bear')
'close on river' is the wordplay.
'close' becomes 'end' (I've seen this before).
'on' says to put letters next to each other.
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'end'+'ure'='ENDURE'
(Other definitions for endure that I've seen before include "Last or suffer patiently" , "Go through" , "Put up with - last" , "Last or undergo" , "survive" .)