Rule applied to criminal faction or one cheating partner (10)

I believe the answer is:

fornicator

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'one cheating partner' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'rule applied to criminal faction or' is the wordplay.
'rule' becomes 'r' (legal abbreviation).
'applied to' is a charade indicator (letters next to each other).
'criminal' indicates anagramming the letters.
'faction'+'or'='factionor'
'factionor' anagrammed gives 'fornicato'.
'r' put after 'fornicato' is 'FORNICATOR'.

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I've seen this clue in The Guardian.

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