Mock free river patrols (6)
I believe the answer is:
deride
'mock' is the definition.
(deriding is a kind of mocking)
'free river patrols' is the wordplay.
I cannot quite see how this works, but
'free' could be 'rid' (to be free of something is to be rid of it) and 'rid' is found in the answer.
'river' could be 'dee' (River Dee) and 'dee' is located in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for deride that I've seen before include "Express contempt for" , "Jeer, belittle" , "Laugh at or make fun of" , "Poke fun at" , "Ridicule, scoff at" .)