Seafood is said to be beef (6)
I believe the answer is:
mussel
'seafood' is the definition.
(I know that mussel is a type of seafood)
'said to be beef' is the wordplay.
'said to be' shows a homophone (sound like).
'beef' becomes 'muscle' (I've seen this before).
'muscle' is a homophone of 'MUSSEL'.
'is' acts as a link.
(Other definitions for mussel that I've seen before include "finds power in sound" , "Black marine bivalve" , "Shellfish sounds like part of the body" , "Type of shellfish" , "Edible marine mollusc sold by Molly Malone - not the cockle" .)