Assign position in river (6)
I believe the answer is:
depute
'assign' is the definition.
(deputing is a kind of assigning)
'position in river' is the wordplay.
'position' becomes 'put' ('put' can be a synonym of 'position'**).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'put' inserted within 'dee' is 'DEPUTE'.
(Other definitions for depute that I've seen before include "Appoint to act on one's behalf" , "Delegate task to someone" , "Delegate to, appoint as representative" .)