Crustacean that lives attached to rocks (8)
I believe the answer is:
barnacle
(Other definitions for barnacle that I've seen before include "Shellfish that sticks to things" , "Marine crustacean (attaching to rocks, ships etc.)" , "Crustacean that attaches to a submerged surface" , "Hull resident (one who clings on)?" , "Be carnal consuming this shellfish" .)