  The CLusters in the Uv as EngineS (CLUES) survey is a Cosmic Origins
Spectrograph (COS) campaign aimed at acquiring the 1130 to 1770 {\AA},
restframe spectroscopy of very young (<20 Myr) and massive (>10^4 solar masses)
star clusters in galaxies that are part of the Hubble treasury program Legacy
ExtraGalactic Uv Survey (LEGUS). In this first paper of a series, we describe
the CLUES sample consisting of 20 young star clusters and report their physical
properties as derived by both multi-wavelength photometry and far-UV (FUV)
spectroscopy with Hubble Space Telescope (HST). Thanks to the synergy of the
two different datasets we build a coherent picture of the diverse stellar
populations found in each region (with sizes of 40 to 160 pc). We associate the
FUV-brightest stellar population to the central targeted star cluster and the
other modeled population to the diffuse stars that are included in the COS
aperture. We observe better agreement between photometric and spectroscopic
ages for star clusters younger than 5 Myr. For clusters older than 5 Myr,
photometry and spectroscopy measurements deviate, with the latter producing
older ages, due to the degeneracy of photometric models. FUV spectroscopy
enables us to better constrain the stellar metallicities, a parameter that
optical colors are insensitive to. Finally, the derived E(B-V) are quite
similar, with a tendency for FUV spectroscopy to favor solutions with higher
extinctions. The recovered masses are in agreement within a factor of 2 for all
the clusters.
