  HESS J1858+020 is a weak gamma-ray source lying near the southern border of
the SNR G35.6-0.4. A molecular cloud, composed by two clumps, shows signs of
interaction with the SNR and with a nearby extended HII region. In particular,
the southernmost clump coincides with the center of the HESS source. In this
work we study this clump in detail with the aim of adding information that
helps in the identification of the nature of the very-high energy emission. We
observed the mentioned molecular clump using the Atacama Submillimeter
Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3 and CS
J=7-6 lines with an angular resolution of 22". To complement this observations
we analyzed IR and submillimeter continuum archival data. From the 12CO and
13CO J=3-2 lines and the 1.1 mm continuum emission we derived a density of
between 10^{3} and 10^{4} cm^-3 for the clump. We discovered a young stellar
object (YSO), probably a high mass protostar, embedded in the molecular clump.
However, we did not observe any evidence of molecular outflows from this YSO
which would reveal the presence of a thermal jet capable of generating the
observed gamma-rays. We conclude that the most probable origin for the TeV
gamma-ray emission are the hadronic interactions between the molecular gas and
the cosmic rays accelerated by the shock front of the SNR G35.6-0.4.
