brendon, have a bit more faith in your own ability, of course you are right with rapier - it fits the clue. you don't need anyone to tell you that the clue has got nothing to do with a wild donkey,do you!
brendan, I didn't know that an onager was any sort of a weapon, but it doesn't fit the clue, only the definition. rapier,as you saw yourself fits the def and the wordplay. sorry if I came over a bit preachy.