Remove permit to enter river (6)
I believe the answer is:
delete
'remove' is the definition.
(deleting is a kind of removing)
'permit to enter river' is the wordplay.
'permit to' becomes 'let' (I've seen this before).
'enter' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'let' placed within 'dee' is 'DELETE'.
(Other definitions for delete that I've seen before include "Erase or destroy" , "Scrub" , "Remove, cross out" , "Eras" , "Strike out (text)" .)