Burrowing mollusc (4)
I believe the answer is:
clam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">burrowing 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 clam that I've seen before include "Tightly closed shellfish" , "Proverbially reticent marine mollusc" , "Mollusc with firmly-closed shell" , "shy person" , "What could grip" .)