Crustacean or, alternatively, lean crab (8)
I believe the answer is:
barnacle
'crustacean' is the definition.
(barnacle is a kind of crustacean)
'alternatively lean crab' is the wordplay.
'alternatively' indicates anagramming the letters (I've seen this in other clues).
'lean'+'crab'='leancrab'
'leancrab' anagrammed gives 'BARNACLE'.
'or' acts as a link.
(Other definitions for barnacle that I've seen before include "Marine crustacean (attaching to rocks, ships etc.)" , "Shellfish that sticks to things" , "Be carnal consuming this shellfish" , "Crustacean attached to rocks" , "kind of goose" .)