Fish in river finds crack (6)
I believe the answer is:
decode
'crack' is the definition.
(I know that crack can be written as decode)
'fish in river' is the wordplay.
'fish' becomes 'cod' (type of fish).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cod' placed inside 'dee' is 'DECODE'.
'finds' is the link.
(Other definitions for decode that I've seen before include "Find hidden meaning" , "Break a cipher" , "Penetrate or solve encrypted message" , "Interpret" , "Get cracking" .)