  Background: The understanding of nuclear reactions between light nuclei at
energies below the Coulomb barrier is important for several astrophysical
processes, but their study poses experimental and theoretical challenges. At
sufficiently low energies, the electrons surrounding the interacting ions
affect the scattering process. Moreover, the clustered structure of some of
these nuclei may play a relevant role on the reaction observables.
  Purpose: In this article, we focus on a theoretical investigation of the role
of clustered configurations of $^6$Li in reactions of astrophysical interest.
  Methods: The $^6$Li(p,$^3$He)$^4$He reaction cross section is described
considering both the direct transfer of a deuteron as a single point-like
particle in Distorted Wave Born Approximation (DWBA), and the transfer of a
neutron and a proton in second-order DWBA. A number of two- and three-cluster
structure models for $^6$Li are compared.
  Results: Within the two-cluster structure model, we explore the impact of the
deformed components in the $^6$Li wave-function on the reaction of interest.
Within the three-cluster structure model, we gauge the degree of $\alpha$-d
clustering and explicitly probe its role on specific features of the reaction
cross section. We compare the energy trend of the astrophysical $S$ factor
deduced in each case.
  Conclusions: Clustered $^6$Li configurations lead in general to a significant
enhancement of the astrophysical factor in the energy region under study. This
effect only originates from clustering, whereas static deformations of the
ground-state configuration play a negligible role at very low energies.
