You definitely do not need an anemone for clowns :) In fact, clowns often do nothing for an anemone at all, they just hangout in the tentacles, loving life. However, anemones can be a fun addition to any reef tank :)
If you have clownfish, read up and once you're ready, definitely get an anemone. The interaction between the clownfish and anemone is what got me into reefkeeping.