  We present and analyse an extensive dataset of the superluminous supernova
(SLSN) LSQ14mo (z = 0.256), consisting of a multi-colour lightcurve from -30 d
to +70 d in the rest-frame and a series of 6 spectra from PESSTO covering -7 d
to +50 d. This is among the densest spectroscopic coverage, and
best-constrained rising lightcurve, for a fast-declining hydrogen-poor SLSN.
The bolometric lightcurve can be reproduced with a millisecond magnetar model
with ~ 4 M_sol ejecta mass, and the temperature and velocity evolution is also
suggestive of a magnetar as the power source. Spectral modelling indicates that
the SN ejected ~ 6 M_sol of CO-rich material with a kinetic energy of ~ 7 x
10^51 erg, and suggests a partially thermalised additional source of luminosity
between -2 d and +22 d. This may be due to interaction with a shell of material
originating from pre-explosion mass loss. We further present a detailed
analysis of the host galaxy system of LSQ14mo. PESSTO and GROND imaging show
three spatially resolved bright regions, and we used the VLT and FORS2 to
obtain a deep (five-hour exposure) spectra of the SN position and the three
star-forming regions, which are at a similar redshift. The FORS spectrum at
+300 days shows no trace of SN emission lines and we place limits on the
strength of [O I] from comparisons with other Ic SNe. The deep spectra provides
a unique chance to investigate spatial variations in the host star-formation
activity and metallicity. The specific star-formation rate is similar in all
three components, as is the presence of a young stellar population. However,
the position of LSQ14mo exhibits a lower metallicity, with 12 + log(O/H) = 8.2
in both the R23 and N2 scales (corresponding to ~ 0.3 Z_sol). We propose that
the three bright regions in the host system are interacting, which thus
triggers star-formation and forms young stellar populations.
