Tosses half of butter for main dish (7)
I believe the answer is:
lobster
'main dish' is the definition.
('the main' is an old name for the sea)
'tosses half of butter' is the wordplay.
'tosses' becomes 'lobs'.
'half of' suggests halving the letters.
'butter' with half the letters taken is 'ter'.
'lobs'+'ter'='LOBSTER'
'for' is the link.
(Other definitions for lobster that I've seen before include "Crustacean with pincer-like claws" , "one for the pot?" , "Large marine decapod" , "food?" , "Large, strong-jawed crustacean" .)