Clinging mollusc (6)
I believe the answer is:
limpet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">clinging mollusc</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)