  Supergiant fast X-ray transients (SFXTs) are a peculiar class of supergiant
high-mass X-ray binary (HMXB) systems characterised by extreme variability in
the X-ray domain. In current models, this is mainly attributed to the clumpy
nature of the stellar wind coupled with gating mechanisms involving the spin
and magnetic field of the neutron star. We studied the X-ray properties of the
supergiant HMXB XMMU J053108.3-690923 in the Large Magellanic Cloud to
understand its nature. We performed a detailed temporal and spectral analysis
of the eROSITA and XMM-Newton data of XMMU J053108.3-690923. We confirm the
putative pulsations previously reported for the source with high confidence,
certifying its nature as a neutron star in orbit with a supergiant companion.
We identify the extremely variable nature of the source in the form of flares
seen in the eROSITA light curves. The source flux exhibits a total dynamic
range of more than three orders of magnitude, which confirms its nature as an
SFXT, and is the first such direct evidence from a HMXB outside our Galaxy
exhibiting a very high dynamic range in luminosity as well as a fast flaring
behaviour. We detect changes in the hardness ratio during the flaring intervals
where the hardness ratio reaches its minimum during the peak of the flare and
increases steeply shortly afterwards. This is also supported by the results of
the spectral analysis carried out at the peak and off-flare intervals. This
scenario is consistent with the presence of dense structures in the supergiant
wind of XMMU J053108.3-690923 where the clumpy medium becomes photoionised at
the peak of the flare leading to a drop in the photo-electric absorption.
Further, we provide an estimate of the clumpiness of the medium and the
magnetic field of the neutron star assuming a spin equilibrium condition.
