Salt in plain shellfish (8)
I believe the answer is:
barnacle
'shellfish' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'salt in plain' is the wordplay.
'salt' becomes 'nacl' (NaCl is the chemical formula for sodium chloride).
'in' is an insertion indicator.
'plain' becomes 'bare' (similar in meaning).
'nacl' placed into 'bare' is 'BARNACLE'.
Can you help me to learn more?
(Other definitions for barnacle that I've seen before include "Clinging crustacean" , "Marine crustacean (attaching to rocks, ships etc.)" , "One that sticks" , "A shellfish that sticks to hulls etc" , "underwater attachment" .)