  We report photometric and spectroscopic observations of the optical transient
LSQ13zm. Historical data reveal the presence of an eruptive episode (that we
label as `2013a') followed by a much brighter outburst (`2013b') three weeks
later, that we argue to be the genuine supernova explosion. This sequence of
events closely resemble those observed for SN2010mc and (in 2012) SN2009ip. The
absolute magnitude reached by LSQ13zm during 2013a
($M_R=-14.87\pm0.25\,\rm{mag}$) is comparable with those of supernova
impostors, while that of the 2013b event ($M_R=-18.46\pm0.21\,\rm{mag}$) is
consistent with those of interacting supernovae. Our spectra reveal the
presence of a dense and structured circumstellar medium, probably produced
through numerous pre-supernova mass-loss events. In addition, we find evidence
for high-velocity ejecta, with a fraction of gas expelled at more than
20000\kms. The spectra of LSQ13zm show remarkable similarity with those of
well-studied core-collapse supernovae. From the analysis of the available
photometric and spectroscopic data, we conclude that we first observed the last
event of an eruptive sequence from a massive star, likely a Luminous Blue
Variable, which a short time later exploded as a core-collapse supernova. The
detailed analysis of archival images suggest that the host galaxy is a
star-forming Blue Dwarf Compact Galaxy.
