Shellfish with physical strength, by the sound of it (6)
I believe the answer is:
mussel
'shellfish' 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' shows a homophone (sound like).
'muscle' is a homophone of 'MUSSEL'.
'with' is the link.
(Other definitions for mussel that I've seen before include "Edible bivalve mollusc, common in sea" , "finds power in sound" , "Black marine bivalve" , "Black marine bivalves" , "Edible bivalve mollusc, sticks to rocks" .)