Find solution to fish in river (6)
I believe the answer is:
decode
'find solution to' is the definition.
I can't tell whether this definition defines the answer.
'fish in river' is the wordplay.
'fish' becomes 'cod' (type of fish).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cod' going within 'dee' is 'DECODE'.
Can you help me to learn more?
(Other definitions for decode that I've seen before include "Put in" , "Convert a message into ordinary language" , "Get cracking" , "Break (a secret message)" , "Interpret" .)