Shell of ruined monastery, deserted by man (6)

I believe the answer is:

oyster

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'shell' is the definition.
(oyster shell is a kind of shell)

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

(Other definitions for oyster that I've seen before include "Parent of the pearl" , "Maybe grey" , "Pearl-yielding shellfish" , "Bivalve that may nurture a pearl" , "Pearl's home?" .)

I've seen this clue in the Sydney Morning Herald.

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