  The single-spin asymmetry of unpolarized leptons scattering
deep-inelastically off transversely polarized nucleons is studied in a partonic
picture within a collinear twist-3 framework. Since this observable is
generated by multi-photon exchanges between lepton and nucleon a partonic
description of the asymmetry contains essential elements of a full
next-to-leading order calculation for single-spin asymmetries in perturbative
Quantum Chromodynamics. In particular it is shown how nontrivial cancellations
between kinematical and dynamical twist-3 contributions lead to a well-behaved
and finite formula. This final result can be expressed in terms of
multipartonic quark-gluon and quark-photon correlation functions. Hence, a
measurement of the transverse target single-spin asymmetry may provide new
constraints on these multipartonic correlations.
