I think CommonReactor's got the right idea. My only suggestion would be to use the definite article with the second, as follows:
"The clown is a modern representation of the harlequin."
"The" in "the clown" indicates a concept familiar to the reader. "a" in "a modern representation" suggests that this is new information - which I believe it is, since the sentence is informative. The indefinite article also leaves open the possibility that there might be more than one modern representation of the harlequin, which makes perfect sense to me. "The" in "the harlequin" suggests that you are accessing a known concept as well... your reader may not be as familiar with harlequins, but the harlequin is an established cultural concept, so it feels right to me. It also forms a nice parallelism with "the clown."
Which is a long way of explaining the reasoning behind my suggestion, but I hope it helps.