Novella Nelson is very hard to pin down. And it's just not the sweep of her credits as singer, actress, producer and director.

When she is singing - even something as exhausted as "Danny Boy" - a seamless world of caprice, hurt, determination and anger gushes from her thin frame and big, imposing voice. When she first appears in the new Alan Bates film "An Unmarried Woman," she looks like a man.

"I can't pin me down and that doesn't worry me," says Nelson, her voice momentarily airy. She quickly switches to a more earnest tone, allowing, "Everyone sees different parts of me. But like the character in the movie, I am a free spirit. She has a grip on things and so do I."

She enjoys the elusiveness and fights any type of categorization. "Ask me in another five months who I am," she says. Right now she is modifying both her professional and personal life. Within the last year she has had a child, assuming all the responsibilities of a single parent, and, after 10 full years in show business, hired an agent. Her music is mellower, she is no longer the searing artist of five years ago who looked for - and spelled out - the irony in every lyric. And the changes seem to exhilarate her, she seems to revel in the process of discovery, even if she doesn't know the outcome.

Recounting the last two years, she pauses, her face suddenly collapsing into worry lines. "Do I look like a man the whole time in the movie?" She is assured that in her film debut, she does have a feminine presence. Her girlish, gap-tootheh smile reappears.

Nelson first gained attention in the musical theater as Madame Tango in the 1967 off-Broadway production of "House of Flowers" and then as Pearl Baileys understudy in "Hello Dolly!" For sometime now, especially since Nina Simone's reclusion, Nelson has been one of the few singers who offer a serious message. Yet her concertion Thursday night at the Smithsonian Institution was much looser than those of a few years ago - she massaged rather than jabbed.

The audience, a predominately female one that's become the underplanning of the Nelson cult, requested some of her elder, strident material.Her show still is a combination of songs and poems reflecting the black and female struggle. When the audience asked for "When Johnny Comes Marching Home," in her hands an anti-war song, and "I Wish I Knew How It Would Feel To Be Free." Nelson performed them but with obvious reluctance.

"Maybe it's because we are in a museum, maybe that's why people are so serious. Me, I'm up, I want to boogie," she said, and evened out her mood with "Jeepers, Creepers."

What her fans want is the dramatic, meaningful definition of familiar songs, such as "Sitting On the Dock of the Bay." She disagrees that she is unique. "I think George Benson, Lou Rawls and Melbor Moore are relating to their words. Most people believe I am not commercial, but I am; everyone in the business is. But the industry looks at me as a performing artist, not a commercial one, and wonders how they can package that energy," says Nelson. She has made only one album, in 1970, for an obscure label.

Music hasn't been her only expression of who Novella Nelson is, or will be. Her other credentials include: the original cast of "Purlie"; the Rex Harrison "Caesar and Cleopatra"; five years with Joseph Papp's Public Theater; regular gigs at new York's best-known, avant-garde clubs, and television.

During her concert Nelson tells the story of an African woman, who walks around the village muttering to herself. Nelson has bent her tall, slim body into that woman's worn form. Then, slowly, Nelson straightens up, adjusting the fluid lines of her purple and white chiffon caftan to its intended softness, and recites, the song, "Eleanor Rigby." Her angular face shows the common bond of pain and confusion.

"How old are you," a woman in the front row shouts out.

Nelson, who is 39, stands still, her eyes narrowed until they are almost closed. "I am not a young girl," she says, "but I am not pretending to be anything but what I am."