Crossword Genius

Tough seafood with starter changed (7)

Ross

I believe the answer is:

mobster

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'tough' is the definition.
(both can mean a violent person)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mobster that I've seen before include "Gangster" , "family man" , "Hoodlum" , "Mafioso" , "Member of criminal organisation" .)

I've seen this clue in The Sun.
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