Gas almost vacated the ancient mollusc
I believe the answer is:
ammonite
'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" .)