Salt swallowed by simple crustacean (8)
I believe the answer is:
barnacle
'crustacean' is the definition.
(I know that Barnacle is a type of crustacean)
'salt swallowed by simple' is the wordplay.
'salt' becomes 'nacl' (NaCl is the chemical formula for sodium chloride).
'swallowed by' indicates putting letters inside.
'simple' becomes 'bare' (synonyms).
'nacl' put into 'bare' is 'BARNACLE'.
(Other definitions for barnacle that I've seen before include "'Marine crustacean, attaches to a boat-hull (8)'" , "Crustacean attached to rocks" , "Marine crustacean (attaching to rocks, ships etc.)" , "Clinging sort" , "A shellfish that sticks to hulls etc" .)