Remove obstruction in river (6)
I believe the answer is:
delete
'remove' is the definition.
(I know that delete is a more specific form of the action remove)
'obstruction in river' is the wordplay.
'obstruction' becomes 'let' (let can mean a hindrance).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'let' put into 'dee' is 'DELETE'.
(Other definitions for delete that I've seen before include "Eras" , "Blot out or erase" , "Wipe out" , "Obliterate" , "Cut, erase" .)