||Contemporary works suggest that the progenitor of solar eruption in the source region is either a magnetic flux rope or sheared arcade. We present a multi-wavelength analysis of a partially erupting filament associated with an M6.6 flare from the active region NOAA 12371. The prominence eruption evolved into a huge, ultra-fast, halo coronal mass ejection. In this study, we combine observations from the Solar Dynamics Observatory (SDO), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Big Bear Solar Observatory (BBSO). GOES 1-8 A soft X-ray (SXR) flux indicates this flare to be a typical long duration event (LDE), causing prolonged thermal emission for >3 hours. The flare light curves in SXR and extreme ultraviolet (EUV) energy bands suggest distinct peaks during pre-flare and precursor phases which are associated with triggering reconnection and heating of a pre-existing quasi-stationary flux rope. Interestingly, the main phase of the flare was associated with two distinct peaks separated by ~15 minutes. The beta-gamma type active region consisted of two spatially well separated sunspot groups with the leading and trailing groups showing unipolar and bipolar magnetic structures on the photosphere, respectively. Chromospheric H-alpha and EUV observations suggest the bipolar sunspot to be associated with a filament lying over the polarity inversion line which partially erupted giving rise to the impulsive emission with strong non-thermal characteristics, causing the first SXR peak. The filament eruption was followed by intense diffused emission from the newly formed post-flare arcade giving rise to the gradual, second SXR peak. The multi-wavelength observations and magnetic field modelling suggest that the magnetic reconnection during the precursor phase leads to the further build-up and activation of a pre-existing magnetic flux rope. We also quantify and discuss the thermal and non-thermal characteristics of the flare emission during the impulsive phase.