It's possible. Clowns are large creatures with lots of natural combat skill, but they aren't invincible. Legends or near-legends in masterwork steel or candy armor can defeat them one one one easily, and even three or four on one with luck. Given that there's only ~50-80 demons in the initial wave, you might even be able to outnumber them if you have a very large army. Also, at least some of those will be made of undefined materials, like grime, snow, or ash, and will thus be extremely fragile, to the extent where you can probably cut the number of "effective" demons in half.
Of course, there are other problems involving meeting the clowns head-on. First, you're more-or-less certain to have part or all of your military afflicted by various nasty syndromes that might take them out even if they manage to survive the actual conflict. Then there's fire demons, who explode into a cloud of hellfire which is far hotter than the sun when killed. One such burst could wipe out your entire military, and if there's one fire demon, there's probably more than one. Unfortunately, crossbowdwarves aren't as effective against the flesh-and-blood or defined-material clowns as one would like, so you can't entirely rely on them to take out the whole host from ranged safety. Basically, you're sure to lose dwarves in a clown encounter. The question is, do you lose more than you can afford? That's uncertain.