Make it plain that there are fish in the river (6)
I believe the answer is:
decode
'make it plain that there' is the definition.
The answer and definition are not the same part of speech.
'fish in the river' is the wordplay.
'fish' becomes 'cod' (type of fish).
'in the' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'cod' placed within 'dee' is 'DECODE'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for decode that I've seen before include "Decipher, decrypt" , "Penetrate or solve encrypted message" , "Unscramble" , "Convert a message into ordinary language" , "Explain" .)