Treacheries scuppered if I listened in, mostly (12)

I believe the answer is:

infidelities

'treacheries' is the definition.
(treacherous or unfaithful acts)

'scuppered if i listened in mostly' is the wordplay.
'scuppered' indicates anagramming the letters ('scuppered' can mean damaged).
'mostly' means to remove the last letter.
'if'+'i'+'listened'+'in'='ifilistenedin'
'ifilistenedin' with its final letter removed is 'ifilistenedi'.
'ifilistenedi' is an anagram of 'INFIDELITIES'.

I've seen this clue in The Times.

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