Definition of a cluster
The properties of a cluster depend on the exact number of constituents, and the geometry of the arrangement.

The properties of a bulk solid otoh are just indifferent to the exact size of the specimen (e.g. crystallite).

(In between you have nanocrystalline substances, where properties change with size, but only gradually.)

I would say that clusters are much like molecular compounds in that they have a distinct structure and absolute composition, however they are held together not by covalent bonds but by polar or dispersive forces (also metallic), and they usually consist of a number of identical particles.