  We present an unprecedented high-resolution \ha\ imaging spectroscopic
observation of a C4.1 flare taken with IBIS on 2011 October 22. The flare
consists of a main circular ribbon that occurred in a parasitic magnetic
configuration and a remote ribbon that was observed by the IBIS. Such a
circular-ribbon flare with a remote brightening is predicted in 3D fan-spine
reconnection but so far has been rarely observed. During the flare impulsive
phase, we define "core" and "halo" structures in the observed ribbon. Examining
the \ha\ emission spectra averaged in the flare core and halo areas, we find
that only those from the flare cores show typical nonthermal electron beam
heating characteristics. These characteristics include: broad and centrally
reversed emission spectra, excess emission in the red wing with regard to the
blue wing (i.e., red asymmetry), and redshifted bisectors of the emission
spectra. We also observe rather quick timescales for the heating (30 s) and
cooling (14--33 s) in the flare core locations. Therefore, we suggest that the
flare cores revealed by IBIS track the sites of electron beam precipitation
with exceptional spatial and temporal resolution. The flare cores show
two-stage motion (a parallel motion along the ribbon followed by an expansion
motion perpendicular to the ribbon) during the two impulsive phases of the
flare. Some cores jump quickly (30 \kms) between discrete magnetic elements
implying reconnection involving different flux tubes. We observe a very high
temporal correlation ($\gtrsim0.9$) between the integrated \ha\ and HXR
emission during the flare impulsive phase. A short time delay (4.6 s) is also
found in the \ha\ emission spikes relative to HXR bursts. The ionization
timescale of the cool chromosphere and the extra time taken for the electrons
to travel to the remote ribbon site may contribute to this delay.
