  Precision measurement of the cross section for single top production is an
important test of the Standard Model (SM). The purity of the collected data in
single top events is limited by the understanding of the shape and yield of
background contributions. Besides electroweak and $\rm{t\bar{t}}$ processes,
QCD multijet events constitute a non-negligible background for the considered
signal bq$'\rightarrow$ tq ($\textit{t}$-channel) process. The data-driven
technique for constraining QCD contribution, employed in the measurement of the
$\textit{t}$-channel single top-quark cross section using the very first LHC
proton-proton collisions at $\sqrt{s}=13\ TeV$ with the CMS detector, is
described. The dataset corresponds to an integrated luminosity of
$\mathcal{L}=42\ pb^{\mathrm{-1}}$.
