Start to roast one's fish croquette (7)
I believe the answer is:
rissole
'croquette' is the definition.
(I've seen this before)
'start to roast one's fish' is the wordplay.
'start to' indicates taking the first letters.
'one's' becomes 'is' (roman numeral).
'fish' becomes 'sole' (I've seen this before**).
The first letter of 'roast' is 'r'.
'r'+'is'+'sole'='RISSOLE'
(Other definitions for rissole that I've seen before include "Spiced meat coated in breadcrumbs and fried" , "Meat product" , "faggot" , "Fried meat dish" , "Losers I find in the chopped meat dish" .)