After three years as Nickelodeon's resident expert on everything teenish, ''Clarissa'' (actually 18-year-old Melissa Joan Hart) of Clarissa Explains It All is applying something other than expertise to a new endeavor: music. This month her new band, Clarissa and the Straightjackets, releases its first CD, This Is What Na Na Means. The nonsensically titled Na Na, produced to sound as if it were recorded during a garage rehearsal, is a cross between the Go-Go's and the Brady Bunch that is, in a good way.
Clarissa/Melissa admits, ''I haven't sung before, so it doesn't sound exactly professional.'' But she tries. For example, on ''O-Brother,'' an ode to Clarissa's annoying TV sibling, Ferguson, Hart takes on a Joan Jett-esque growl. She then goes into an alternative rock-style lament on ''Wishing for Rain.'' And despite very little vocal flexibility, she does pull off one track that's much less adolescent than the others: ''P.E.A.C.E.,'' a song about a friend whose parents argue constantly.
On top of her new career in music, Hart will be enrolled as a part-time liberal-arts freshman at New York University this fall. And how will this college student balance the pressures of school with the demands of both an acting and a singing career? That, even for Clarissa, may be hard to explain.