Clear to enter river in Knock? (6)
I believe the answer is:
deride
'knock?' is the definition.
(I've seen this in another clue)
'clear to enter river' is the wordplay.
'clear' becomes 'rid' (synonyms).
'to enter' indicates putting letters inside.
'river' becomes 'Dee' (River Dee).
'rid' inserted inside 'dee' is 'DERIDE'.
'in' is the link.
(Other definitions for deride that I've seen before include "Jeer, belittle" , "Laugh at or make fun of" , "Ridicule, scoff at" , "Express contempt for" , "Poke fun at" .)