It’s free to be immersed in river in Knock (6)
I believe the answer is:
deride
'knock' is the definition.
(I've seen this before)
'it's free to be immersed in river' is the wordplay.
'it's 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' put into 'dee' is 'DERIDE'.
'in' is the link.
(Other definitions for deride that I've seen before include "Disparage" , "Mock, scorn" , "'Mock, sneer at (6)'" , "Ridicule, scoff at" , "Knock" .)