Ace fish wraps in cooker (7)
I believe the answer is:
costard
'cooker' is the definition.
(type of cooking apple)
'ace fish wraps' is the wordplay.
'ace' becomes 'star' (synonyms).
'fish' becomes 'cod' (type of fish).
'wraps' means one lot of letters goes inside another (inserted letters get wrapped up).
'star' going within 'cod' is 'COSTARD'.
'in' is the link.
(Other definitions for costard that I've seen before include "Clown" , "Cooking apple" , "Fruit" , "cooker" .)