In this paper we employ hydrodynamic formulation of time-dependent density-functional theory to obtain coefficient C-6 of the long-range part of the van der Waals interaction between alkali-metal clusters of large sizes. Such a calculation becomes computationally very demanding in the orbital-based Kohn-Sham formalism, but is quite simple in the hydrodynamic approach. This is because in hydrodynamic formulation, electron density and current density, rather than the orbitals, are employed as basic variables. We show that for intercations between the clusters of same sizes, C-6 scales as the sixth power of the cluster radius and approaches the classically predicted value for large size clusters.