Certain molluscs are in badly behaved manner, I hear (7)

I believe the answer is:

nautili

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'certain molluscs' is the definition.
'nautili' can be an answer for 'molluscs' (nautilus is a kind of mollusc). I'm not sure about the 'certain' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for nautili that I've seen before include "Cephalopod molluscs" , "creatures in the sea" , "marine life" .)

I've seen this clue in the Sydney Morning Herald.

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