Free to be immersed in river in Knock (6)
I believe the answer is:
deride
'knock' is the definition.
(I've seen this in another clue)
'free to be immersed in river' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'to be immersed in' means one lot of letters goes inside another.
'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 "'Mock, sneer at (6)'" , "Express contempt for" , "Disparage" , "Jeer, make little of" , "Jeer, belittle" .)