Snail or slug, eg (9)

I believe the answer is:

gastropod

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'snail or slug' is the definition.
(I've seen this in another clue)

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

(Other definitions for gastropod that I've seen before include "Slimy type" , "slug, perhaps" , "Mollusc such as snail or limpet" , "Shellfish" , "Could be snail" .)

I've seen this clue in The Times.

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