Cancel river crossing permit (6)
I believe the answer is:
delete
'cancel' is the definition.
(I've seen this before)
'river crossing permit' is the wordplay.
'river' becomes 'dee' (River Dee).
'crossing' is an insertion indicator.
'permit' becomes 'let' ('let' can be a synonym of 'permit').
'dee' going around 'let' is 'DELETE'.
(Other definitions for delete that I've seen before include "'Cancel, cut (6)'" , "Take out" , "Cut, erase" , "Erase or destroy" , "Cancel, erase" .)