I've never had a clown knife myself, from what I've read this is one of those may/may not work combo's. The only way you'll know is to try it out. It is important though that you have multiple large tanks, so if there is a problem you can quickly seperate the two fish.
Anyways, here's a pic of actual aro/knife/gar combo
My aro and ck "spar" every so often but there's never any damage beyond a scratch or nipped fin. I also have a royal and a bronze knife in with my Green aro.