In general the two species of fish should get along. I would like to ask you a few questions though:
1.How big is your aquarium (not including sumps etc.)
2.What kind of Clown fish do you have?
3.What kind of Tang do you have?
4.How many of each do you have in the aquarium?
5.How much actual space do you have available in your aquarium for these specimens?
6.What is the diet of these specimen and how often are they fed?
7.Which one was in the tank first?
If you would answer these questions for me I will be able to better assist you with this issue.