How can I apply the theory of feminism to the poem "There is a Girl Inside of Me " ?
Lucille Clifton is at her best, I think, when she's writing about the full range of experiences of the female body, both good and bad. Her poem "There is a girl inside" may seem at first to be quite different from her poems showing a female speaker who loves her own big hips or feels grief over her abortions, but it in fact it is much like them in how it, too, is closely tied to a woman's life experiences.
There is no one theory of feminism, of course, and the types of feminism that might best apply to "There is a girl inside" are perhaps ones (often called "radical feminisms" or "essentialist feminisms") that embrace the female body and see it as the foundation of female identity. Adrienne Rich and Monique Wittig may represent this sort of feminism.
The poem itself seems to challenge stereotypes about what girls and women should be, perhaps even by literally rewriting the fairy tale of Little Red Riding Hood, which has often been taken as a tale of female maturation (e.g. the red hood might symbolize menstrual blood and the wolf might symbolize rampant sexual desire). Indeed, the open lines of Clifton's poem calls this girl "randy as a wolf" and very different from a grandmother. If you follow this perspective, you may find it helpful to compare Clifton's poem to Angela Carter's prose rewritings of the same fairy tale.
The speaker in this poem is no longer willing to wait "patient as a nun." She embraces her sexuality, curses, has multiple lovers, and so on, but these developments are hardly presented as destructive. They afford her the opportunity to fully explore herself and her world.