12 across
Heretics
are
punished
for
unfaithful
acts
(11)
Heretics are punished for unfaithful acts (11)
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The definition is at the end of the clue.
How the clue works
Unfaithful acts is your definition — think betrayals. Then "punished" signals an anagram, and sure enough the letters of Heretics are can be rearranged to give you your answer.