Burrowing mollusc (4)
I believe the answer is:
clam
'burrowing mollusc' is the definition.
(I've seen this before)
This is all the clue.
(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" .)