Based on the three-step model, we theoretically studied the energy distribution of low-energy (less than 50eV) secondary electrons ernitted from highly-oriented pyrolytic graphite (HOPG), induced by the 0.24 MeV/atom carbon-cluster C-n(+) (n = 1, 2,4, 8) ions. Here we treated the kinetic excitation process of an electron gas, described by the dynamical dielectric function. In the projectile system, we took into account the charge state, the spatial structure of the cluster and the size of the electron cloud on the constituent atoms. The numerical results show that the intensity of energy spectra per carbon ion is less than that induced by a single carbon ion at the equivalent speed. This sub-linear effect was supported by a recent experimental result. We concluded that this effect could be explained to a considerable extent by the screening effect and the vicinage effect of the incident cluster.