  The detection of Keplerian rotation is rare among Class 0 protostellar
systems. We have investigated the high-density tracer HCN as a probe of the
inner disk in a Class 0 proto-brown dwarf candidate. Our ALMA high angular
resolution observations show the peak in the HCN (3-2) line emission arises
from a compact component near the proto-brown dwarf with a small bar-like
structure and a deconvolved size of $\sim$50 au. Radiative transfer modelling
indicates that this HCN feature is tracing the innermost, dense regions in the
proto-brown dwarf where a small Keplerian disk is expected to be present. The
limited velocity resolution of the observations, however, makes it difficult to
confirm the rotational kinematics of this feature. A brightening in the HCN
emission towards the core center suggests that HCN can survive in the gas phase
in the inner, dense regions of the proto-brown dwarf. In contrast, modelling of
the HCO$^{+}$ (3-2) line emission indicates that it originates from the outer
pseudo-disk/envelope region and is centrally depleted. HCN line emission can
reveal the small-scale structures and can be an efficient observational tool to
study the inner disk properties in such faint compact objects where spatially
resolving the disk is nearly impossible.
