Said why tucking into crab dish could be a danger, in the main (9)

I believe the answer is:

charybdis

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'main' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for charybdis that I've seen before include "Mythical whirlpool" , "Whirlpool opposite Scylla" , "monster" , "Scylla's counterpart" , "Whirlpool, across from 14" .)

I've seen this clue in The Guardian.

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