  In this paper, we present a comprehensive study of the evolutionary phases of
a major M6.6 long duration event (LDE) with special emphasize on its pre-flare
phase. The event occurred in NOAA 12371 on 2015 June 22. A remarkable aspect of
the event was an active pre-flare phase lasting for about an hour during which
a hot EUV coronal channel was in build-up stage and displayed co-spatial hard
X-ray (HXR) emission up to energies of 25 keV. As such, this is the first
evidence of HXR coronal channel. The coronal magnetic field configuration based
on NLFFF modeling clearly exhibited a magnetic flux rope (MFR) oriented along
the polarity inversion line (PIL) and co-spatial with the coronal channel. We
observed significant changes in the AR's photospheric magnetic field during an
extended period of $\approx$42 hours in the form of rotation of sunspots,
moving magnetic features, and flux cancellation along the PIL. Prior to the
flare onset, the MFR underwent a slow rise phase ($\approx$14 km s$^{-1}$) for
$\approx$12 min which we attribute to the faster build-up and activation of the
MFR by tether-cutting reconnection occurring at multiple locations along the
MFR itself. The sudden transition in the kinematic evolution of the MFR from
the phase of slow to fast rise ($\approx$109 km s$^{-1}$ with acceleration
$\approx$110 m s$^{-2}$) precisely divides the pre-flare and impulsive phase of
the flare, which points toward the feedback process between the early dynamics
of the eruption and the strength of the flare magnetic reconnection.
