Maya Angelou's poem "Caged Bird" is full of avian metaphors and imagery. The poem itself is a metaphor for the limitations one experiences in a life of oppression. "Caged Bird" draws from Angelou's own experiences as a Black woman in the racially-segregated United States following the Civil War and even beyond the Civil Rights Movement. To this day, many Black Americans face limitations based on a systemic cycle of racial oppression which prevents class mobility. 

In talking of birds, "clipping" involves trimming a bird's wing feathers so that they cannot fly. Some bird owners or caretakers trim just one wing or enough feathers on each side, so as to render the bird unstable in flight but leaving them able to glide for a short distance. In Angelou's poem, she uses "clipped" as a metaphor for the systemic forms of oppression I have mentioned above. Being "clipped" in society on the basis of race (or other identities) prevents an individual from ever testing their capability for success. Historically, Black Americans have been denied access to schooling and certain kinds of work, and even today it is not uncommon for Black Americans to be turned down for jobs on the basis of their appearance and a failure to assimilate to a white-dominated work environment. To be "clipped," as Angelou implies, is to never be given a chance for success in life.