Shellfish that's shucked (6)
I believe the answer is:
oyster
'shellfish that's shucked' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oyster that I've seen before include "Secretive person; bivalve" , "Succulent bivalve swallowed quickly" , "Mollusc with shucks" , "taken in by carpenter" , "Pearl-bearing sea creature" .)