It depends on the type of nem. Only certain nems will form a symbiotic relationship with certain clownfish and even if you get two compatible ones it isn't always guaranteed that the fish will be hosted by the nem. Some people will have a clown that never pairs up with the anemone in the tank. I had a Sunset Mocha clownfish and it never even once tried to go anywhere near my bubble-tip anemone.
You can definitely keep clowns in a tank without a nem, and you can sometimes keep a nem in a reef tank. Anemones can be frustrating in that they will move if they are not happy in a certain spot, and they can potentially mess up the corals they come into contact with during their journey. Some grow truly massive while others stay relatively small. Some tend to stay put and others like to wander. And sometimes you'll get a nem that stays in one spot for months and then you come home to a mess in the tank because it just decided start moving around one day and got pulled up and diced by one of your wavemakers
It's because of this uncertainty in their nature that one has to really think about if they want to really commit to having an anemone in their tank and understand all of the risks they are taking.
