Crook crustacean with a head twenty times as big (7)

I believe the answer is:

mobster

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'crook' is the definition.
(I know that mobster is a type of crook)

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

(Other definitions for mobster that I've seen before include "family man" , "Hood" , "Criminal gang member" , "Member of criminal organisation" , "Gangster" .)

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

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