Fish in box stuffing part of bird (7)
I believe the answer is:
whiting
'fish' is the definition.
(whiting is a kind of fish)
'box stuffing part of bird' is the wordplay.
'box' becomes 'hit' (I've seen this in other clues).
'stuffing' indicates putting letters inside.
'part of bird' becomes 'wing' ('wing' is part of 'bird').
'hit' put within 'wing' is 'WHITING'.
'in' acts as a link.
(Other definitions for whiting that I've seen before include "Small sea fish or ground chalk for painting" , "Edible marine fish" , "North Atlantic food fish" , "[SOMETHING FISHY]" , "Small fish related to cod" .)