Oooo.... sounds like you need two gags one for the protagonist and one of the antagonist, or does the antagonist already have one?
Actually come to think of it, we need alot more information... what is the skill set and skill level of your actor (not as a actor but as a clown)? What kind of clown is the character? What is the character of the clown character? What kind of props and equitment do you have access to? Where is the gag going to be set?
Then again, a really simple one I have read about and just havent tired yet but sounds like something you might like is a gag found in the back of
Be a clown by Mark Stolzberg.
It is a one man gag, where you parody a high wire act with two chairs and a rope may ten feet long. It begins by setting the two chairs about two feet longer then the rope. Then you tie the rope to the back of one chair (call it chair A), head to the other (chair B) when you nearly reach it you will run out of rope and chair A falls over. You go back to set it up, you set it up go to chair B, A falls over again. A third time. The fourth time you can drag the chair by the end of the rope after it falls over until you can tie it to chair B. Then you look really proud of your self, strut over to chair A, set it up and Chair B falls down. Go to Chair B set it up, A falls down. Repeat this faster and faster until you can't, and fall over into a exahusted heap. Then you un-tie both chairs, make a big show of measuring the distance with your feet, and set the chairs so that the rope will run across, with out the chairs falling over. Then show everyone with great fan fare that you are going to now walk across the "wire." Make a big show of steping on to one of the chairs, steping on to the back and then stepping on to the rope... which promtly falls to the floor bringing the chairs and you with it. You look around bewildered. You pick yourself up, dust yourself off look at the mess and scratch your head. You pick up the chairs, and then press on the rope (or if you are really daring you could try it again but faster with the same result, in fact this might be better) and when the chairs start to come down let go. Plainly this won't work. So, you get an idea and move the chairs closer and closer together checking to see whether the chairs move under a little pressure. They will until the rope is on the floor. Then you do the grandios thing step on to the chair, on to the back and on to the rope and do a mini-highwire act, balancing, juggling, almost falling, a hand stand whatever the actor can do. Eventually you get to chair B climb over it, and take your bows. Then that is pretty much it... though for a blow off (the surprise ending) you could turn around to walk off the stage trip over the "wire" on the floor and carry it and the chairs off stage with your legs at a embarrased and much hindered run.
That is how it goes in my head. I read it in the book a couple of years ago, so it is probably slightly different there. Let the actor play with it all he wants. It has to be real to him to be funny, it has to be his, and it has to be funny, and people on the set should be laughing or it won't work.
Good luck!