Make plain that the river contains fish (6)
I believe the answer is:
decode
'make plain' is the definition.
The answer and definition can be both to do with communicating as well as being verbs in their base form.
Perhaps there's an association between them I don't understand?
'the river contains fish' is the wordplay.
'the river' becomes 'dee' (River Dee).
'contains' means one lot of letters goes inside another.
'fish' becomes 'cod' (type of fish).
'dee' enclosing 'cod' is 'DECODE'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for decode that I've seen before include "Break a cipher" , "Interpret" , "Explain" , "Put in" , "Penetrate or solve encrypted message" .)