  We describe the observations of the low-metallicity nearby galaxy AGC198691
(Leoncino dwarf) obtained with the Integral Field Unit of the instrument MEGARA
at the Gran Telescopio Canarias. The observations cover the wavelength ranges
4304 - 5198 A and 6098 - 7306 A with a resolving power R ~ 6000. We present 2D
maps of the ionized gas, deriving the extension of the HII region and gas
kinematics from the observed emission lines. We have not found any evidence of
recent gas infall or loss of metals by means of outflows. This result is
supported by the closed-box model predictions, consistent with the oxygen
abundance found by other authors in this galaxy and points towards Leoncino
being a genuine XMD galaxy. We present for the first time spatially resolved
spectroscopy allowing the detailed study of a star forming region. We use
PopStar+Cloudy models to simulate the emission-line spectrum. We find that the
central emission line spectrum can be explained by a single young ionizing
cluster with an age ~ 3.5 +/- 0.5Myr and a stellar mass of about 2000 solar
masses. However, the radial profiles of [OIII]5007 A and the Balmer lines in
emission demand photoionization by clusters of different ages between 3.5 and
6.5Myr that might respond either to the evolution of a single cluster evolving
along the cooling time of the nebula (about 3Myr at the metallicity of
Leoncino, Z ~ 0.0004) or to mass segregation of the cluster, being both
scenarios consistent with the observed equivalent widths of the Balmer lines
