Remove obstruction blocking river (6)
I believe the answer is:
delete
'remove' is the definition.
(deleting is a kind of removing)
'obstruction blocking river' is the wordplay.
'obstruction' becomes 'let' (I've seen this in other clues).
'blocking' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'let' placed inside 'dee' is 'DELETE'.
(Other definitions for delete that I've seen before include "Get rid of" , "Scrub" , "Wipe out" , "Key" , "Edit out" .)