Crack fish found in river (6)
I believe the answer is:
decode
'crack' is the definition.
(I know that crack can be written as decode)
'fish found in river' is the wordplay.
'fish' becomes 'cod' (type of fish).
'found in' indicates putting letters inside.
'river' becomes 'Dee' (River Dee).
'cod' placed within 'dee' is 'DECODE'.
(Other definitions for decode that I've seen before include "Penetrate or solve encrypted message" , "Do translation" , "Convert a message into ordinary language" , "Get cracking" , "Break a cipher" .)