  We present observations of SN 2021csp, a unique supernova (SN) which displays
evidence for interaction with H- and He- poor circumstellar material (CSM) at
early times. Using high-cadence spectroscopy taken over the first week after
explosion, we show that the spectra of SN 2021csp are dominated by C III lines
with a velocity of 1800 km s$^{-1}$. We associate this emission with CSM lost
by the progenitor prior to explosion. Subsequently, the SN displays narrow He
lines before metamorphosing into a broad-lined Type Ic SN. We model the
bolometric light curve of SN 2021csp, and show that it is consistent with the
energetic ($4\times10^{51}$ erg) explosion of a stripped star, producing 0.4
M$_\odot$ of 56Ni within a $\sim$1 M$_\odot$ shell of CSM extending out to 400
R$_\odot$.
