Remove obstacle found in river (6)
I believe the answer is:
delete
'remove' is the definition.
(I know that delete is a more specific form of remove)
'obstacle found in river' is the wordplay.
'obstacle' becomes 'let' ('let' could historically mean an obstruction).
'found in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'let' going inside 'dee' is 'DELETE'.
(Other definitions for delete that I've seen before include "Take out" , "Scrub" , "Key" , "Strike out (text)" , "Erase or destroy" .)