Unexploded missiles found on the beach (6)

I believe the answer is:

shells

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'unexploded missiles' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for shells that I've seen before include "Carapaces or military missiles" , "Carapaces or large missiles" , "Hard protective structures" , "Hard outer coatings or metal casings" , "Racing boats" .)

I've seen this clue in the Evening Standard.

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