River fish one's found inside croquette (7)
I believe the answer is:
rissole
'croquette' is the definition.
(I've seen this before)
'river fish one's found inside' is the wordplay.
'river' becomes 'r'.
'fish' becomes 'sole' (I've seen this before**).
'one's' becomes 'is' (Roman numeral).
'found inside' is an insertion indicator.
'r'+'sole'='rsole'
'rsole' going around 'is' is 'RISSOLE'.
(Other definitions for rissole that I've seen before include "Spiced meat coated in breadcrumbs and fried" , "faggot" , "Fried small cake of chopped meat" , "Small pastry" , "Meatball fried in crumbs" .)