There's no need for a Fleet enema or any sort of enema that's high volume or contains chemicals.
You have two options here: taking an enema or not taking an enema.
Some people find that they never need to take an enema or only rarely need to take an enema before "ordinary" anal sex. Before attempting anal play the person who will be receiving can "check themselves out" with a finger or dildo to see if there's anything in the rectum. If not and they're comfortable then you can move straight to anal play.
If there's some stuff in there or you're just more comfortable using an enema then use warm water or warm water with a pinch of salt in it - no soap, no commercial enema solutions, no other odd things. You shouldn't need more than about a cup, and you can repeat it two or three times (you don't need to hold the water in). You can either get a enema bulb or use the Fleet bottles once they've been emptied.
While some may be against the use of enemas at all, I feel that if you are uncomfortable with the thought of trying anal without one you should go ahead. While they can detect changes in the rectal mucosa from enema use, small water or saline enemas don't have a major impact and the rectal mucosa recovers quickly (keep in mind that studies also have found changes in the rectal mucosa from lubricant usage in anal sex). If you are absolutely comfortable with your body and your partner you will be able to relax fully and have a much better chance of having a successful/enjoyable anal experience than if you are worried. Do make sure to plan so that you're trying anal at a time when your intestines aren't likely to be moving stuff around (don't enema, then eat, then try anal - instead eat, take a break, enema, take a break for a movie/foreplay for an hour or so, then try anal), take a break after the enema for 1-2 hours to let the water be absorbed and everything settle down, then use plenty of lube.