  We calculate, within massless QCD, a two-point correlator of nonlocal
(composite) vector quark currents with arbitrary-length chains of the simplest
fermion loops being inserted into gluon lines. Within the large $n_f$ (or large
$\beta_0$) approximation, the correlator defines a perturbative contribution to
the leading-twist distribution amplitudes for light mesons. Our results are
consistent with a number of special cases in the literature. We consider
functionals of the correlator, which are important for the phenomenology, and
their properties as function series.
