Wicked deeds within heartless Mafia factions (8)

I believe the answer is:

infamies

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

'heartless mafia factions' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.

'wicked deeds within' is the wordplay.
I cannot really see how this works, but
'within' could be 'in' and 'in' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Independent.

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