'Dead' isn't a bad answer, but it does have complications. Obviously it will dissuade Alistair from going after Darkling, but it's not going to do Arthur much good.
Then there's the fact that Alistair might already know ZackGrain is a trickster and therefore liable to stretch the truth.
By process of elimination in Dark's party, Alistair, if he isn't busy being totally nuts, would probably infer that he's the trickster who cast Arthur's illusion.
If anything, I expect that Claw hearing that her mortal buddies are dead would prompt her to kill Alistair.