IF YOU'RE SEEKING THE DEFINITIVE viewpoint on teenage girldom, look no further than a rambling, 11-room house in Sayville, N.Y. There, in her hours away from the Orlando, Fla., set where she works, Melissa Joan Hart, 15 and 5' tall, contemplates her spunky TV persona, the title character on Nickelodeon's lauded weekly series Clarissa Explains It All. The wholesome but wisecracking Clarissa, 14, is "a good role model," says Melissa, "because she shows kids not to give in to peer pressure. I like her. Our rooms look the same. We like a lot of the same music, except she likes They Might Be Giants more than I do. My favorite is R.E.M. They're awesome."

So is Melissa's résumé. She began performing at age 4 and at 13 appeared off-Broadway as William Hurt's flashback young love in Beside Herself; she has also been seen in Rice Krispies commercials and on Saturday Night Live. Rich Ross, Nickelodeon's vice president of talent relations, says Melissa was cast as Clarissa because "she's a little quirky, but not put-offish. There's not a jaded quality about her."

Premature ennui has little chance at the Hart house. There, Melissa's father, William, a lobster wholesaler, and mother Paula, a show-business manager for children, preside over their eldest daughter and her four acting siblings: Trisha, 12, Elizabeth, 10, Brian, 7, and Emily, 5.

When not taping Clarissa (in Florida, she stays in a Nickelodeon-owned condo with Mom or a nanny), the ninth grader at Sayville Junior High baby-sits ("I get $3 an hour") and indulges her tastes in ice cream (chocolate), TV (The Simpsons) and movies (Winona Ryder's).

This month Melissa will get her Florida driver's permit. You won't see that on TV: "They let Clarissa get away with a lot of stuff," says Melissa, "but not driving a car."