Lean crab (anag) — one with a hard shell (8)

I believe the answer is:

barnacle

(Other definitions for barnacle that I've seen before include "underwater attachment" , "Crustacean that adheres to ships' bottom" , "Crustacean that attaches to underwater surfaces" , "Shellfish that sticks to things" , "Crustacean that sticks to rocks etc" .)

I've seen this clue in The Guardian.

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