Clinging mollusc (6)
I believe the answer is:
limpet
'clinging mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for limpet that I've seen before include "Gastropod that sticks to rocks" , "A shellfish that sticks to rocks or ships" , "Rock lover" , "Shellfish that sticks to surface" , "Creature that clings to seaside stones" .)