Throws half of butter into shellfish (7)
I believe the answer is:
lobster
'shellfish' is the definition.
(lobster is a kind of shellfish)
'throws half of butter' is the wordplay.
'throws' becomes 'lobs' (lobbing is a kind of throwing).
'half of' suggests halving the letters.
'butter' halved is 'ter'.
'lobs'+'ter'='LOBSTER'
'into' is the link.
(Other definitions for lobster that I've seen before include "Edible marine crustacean" , "Lets bro have the shellfish" , "could find this on the table" , "Bolster (anag.)" , "Tommy no longer" .)