  Nuclear clustering plays an important role, especially in the dynamics of
light nuclei. The importance of the emergence of the nuclear clustering was
discussed in the recent measurement of the photoabsorption cross sections as it
offered the possibility of the coexistence of various excitation modes which
are closely related to the nuclear clustering. To understand the excitation
mechanism, we study the electric-dipole ($E1$) responses of $^6$Li with a fully
microscopic six-body calculation. The ground-state wave function is accurately
described with a superposition of correlated Gaussian (CG) functions with the
aid of the stochastic variational method. The final-state wave functions are
also expressed by a number of the CG functions including important
configurations to describe the six-body continuum states excited by the $E1$
field. We found that the out-of-phase transitions occur due to the oscillations
of the valence nucleons against the $^4$He cluster at the low energies around
10 MeV indicating ``soft'' giant-dipole-resonance(GDR)-type excitations, which
are very unique in the $^6$Li system but could be found in other nuclear
systems. At the high energies beyond $\sim 30$ MeV typical GDR-type transitions
occur. The $^3$He-$^3$H clustering plays an important role to the GDR phenomena
in the intermediate energy regions around 20 MeV.
