Best in fish for cooker (7)
I believe the answer is:
costard
'cooker' is the definition.
(type of cooking apple)
'best in fish' is the wordplay.
'best' becomes 'star' (star can mean the best or most excellent).
'in' indicates putting letters inside.
'fish' becomes 'cod' (type of fish).
'star' inserted inside 'cod' is 'COSTARD'.
'for' acts as a link.
(Other definitions for costard that I've seen before include "cooker" , "Cooking apple" , "Fruit" , "Clown" .)