Shellfish muscle unique (7)
I believe the answer is:
abalone
'shellfish' is the definition.
(abalone is a kind of shellfish)
'muscle unique' is the wordplay.
'muscle' becomes 'ab' (short for abdominal muscle).
'unique' becomes 'alone' (I've seen this in another clue).
'ab'+'alone'='ABALONE'
(Other definitions for abalone that I've seen before include "Ormer, ear shell" , "Mollusc with an oval shell lined with bluish mother-of-pearl" , "produces an ear shell" , "Ear-shaped shell" , "Would you be able on a US shellfish?" .)