ComposHat It depends.
Say your hypothetical person is Pete Doherty, then he may be unlikely to sue even if you were lying.
I'm not sure of Pete Doherty's current drug status, or if papers are interested in him any more, but he probably wouldn't care about a false allegation of drug-taking. Though I imagine he would care if he was accused of abusing children - there are libels and libels.
Plus you might claim justification (that the story was true) which is a complete defence. And you might be able tell a convincing enough tale to the jury, who might decide that given Doherty's reputation that you were telling the truth.
It's a lot of mights, though. And you wouldn't be able to just say it. You would have to provide some proof or corroborating details. And a newspaper would demand that.
But, and here's a money-making tip
, if you were the sort of scoundrel who makes up stories for money, you might want to try one of those true life magazines because the training standards of their staff are appalling and their legal departments aren't great either, if they exist at all.
You could say Hitler was hiding in your basement and it wouldn't occur to them that he would be a bit old now.
They wouldn't want that story btw. Not unless your daughter had been engaged to Hitler but your wife had shacked up in the basement with him too.
But say your hypothetical celebrity was Eric Clapton. He was a junkie but has been clean for 30+ years. Not only that, he's heavily involved in addiction charities.
He would sue your lying arse off and he'd be right. And when he won, the damages would be staggering, because you'd not only be accusing him of taking drugs, which most people dislike, but accept happens; you'd be accusing him of terrible hypocrisy, which I think most people think is worse.
I don't know whether Charles Kennedy would have the stomach for a fight. It's expensive and gruelling and the odds are stacked against the complainant. I never used to think like that but I'm coming round to that idea. Although I remember the bonanza time when a soap actor only had to turn up in court and say: 'The Sun said I was boring. Boo hoo,' and the jury awarded him silly money.
I do know that Kennedy has sworn off alcohol. I saw him politely leave a table when the drinks arrived, presumably because he wants to get well and can't trust himself around alcohol.
So in that case, you'd probably have a hard time proving your lie because if I heard about it I'd be a character witness for him and so would all the other people on that table.
I'm not a libel lawyer btw. Just an unreliable narrator.