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Gas almost vacated the ancient mollusc

Ross

I believe the answer is:

ammonite

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'mollusc' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for ammonite that I've seen before include "A victim of Saul" , "old enemy of Israelites" , "shelled to extinction?" , "old nomadic tribesman" , "Type of fossil" .)

I've seen this clue in The Daily Mail.
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