Remove permit found in river (6)
I believe the answer is:
delete
'remove' is the definition.
(I know that delete is a kind of remove)
'permit found in river' is the wordplay.
'permit' becomes 'let' (synonyms).
'found in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'let' put within 'dee' is 'DELETE'.
(Other definitions for delete that I've seen before include "'Erase, remove (6)'" , "Cancel, erase" , "Take out" , "Rub out, expunge" , "Strike out (text)" .)