Seafood will provide physical strength, by the sound of it (6)
I believe the answer is:
mussel
'seafood' is the definition.
(I know that mussel is a type of shellfish)
'physical strength by the sound of it' is the wordplay.
'physical strength' becomes 'muscle' (I've seen this in another clue).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'muscle' is a homophone of 'MUSSEL'.
'will provide' is the link.
(Other definitions for mussel that I've seen before include "Edible bivalve mollusc, sticks to rocks" , "Shellfish sounds like part of the body" , "Black marine bivalves" , "Edible bivalve mollusc, common in sea" , "finds power in sound" .)