"Spicy world of NanoScience"
Best example is gold nano-clusters (see the JACS publication) - clusters may have fluoresence properties which nano-particles do not have.
I however agree with you that we never find a formal definition of nanoclustes - can we define it as a organized cluster of nanoparticles, which are constrained and partially confined by a matrix ( e.g. a polymeric backbone) . It is such confinements that provides the emerging properties.