Crack fish in river (6)
I believe the answer is:
decode
'crack' is the definition.
(I've seen this before)
'fish in river' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cod' going within 'dee' is 'DECODE'.
(Other definitions for decode that I've seen before include "Break (a secret message)" , "Break a cipher" , "Find hidden meaning" , "Get cracking" , "Crack (a secret message)" .)