Crossword Genius

Metro for the criminal world! (3,11)

Ross

I believe the answer is:

the underground

I'm a little stuck... Click here to teach me more about this clue!

'criminal world' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'metro for the' is the wordplay.
'metro' becomes 'underground' (synonyms).
'for' says to put letters next to each other (I've seen this in other clues).
'underground' after 'the' is 'THE UNDERGROUND'.

Can you help me to learn more?

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