25 across
Attila
the
Hun
originally
behind
old
curse
(4)
Attila the Hun originally behind old curse (4)
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The definition is at the end of the clue.
How the clue works
Look at curse at the end — that's your definition, a swear word. Take the initial letters of Attila the Hun to get ATH, then pop old (abbreviated to O) in front of it, and you've got your answer.