King admitted to engaging in darn cheating (8)

I believe the answer is:

screwing

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'cheating' is the definition.
('screw' can be a synonym of 'cheat')

'king admitted to engaging in darn' is the wordplay.
'king' becomes 'CR' (short for Carolus Rex, King Charles in Latin).
'admitted to' means one lot of letters goes inside another.
'engaging in darn' becomes 'sewing' (I am not sure about this - if you are sure you should believe this answer much more).
'cr' inserted within 'sewing' is 'SCREWING'.

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I've seen this clue in The Times and The Sunday Times.

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