Losing head, violently destroy marine mollusc (6)

I believe the answer is:

oyster

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'marine mollusc' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for oyster that I've seen before include "It produces pearls" , "World as place of opportunity" , "Succulent bivalve, may bear a pearl" , "Parent of the pearl" , "Type of mollusc" .)

I've seen this clue in The Times.

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