In an attempt to analyse how the properties evolve with the cluster size, we report the results of a first principle density functional study on the electronic structures and properties of (BCN)$_x$ clusters (where x = 1 to 5) as well as (BCN)$_{12}$ nanotube. We have investigated geometries of (BCN)$_x$ and their isomers at the B3LYP/6-311G(d) level of theory and their vibrational as well as optical spectra. Their relative stability is discussed by calculating the binding energies. The electronic properties of (BCN)$_x$ clusters and the nanotube are analysed by the frontier orbitals and density of state curves. The frontier orbital energy gap of (BCN)$_{12}$ nanotube is found to be comparable to carbon nanotube but much small as compared to BN nanotube. Various electronic parameters of (BCN)$_x$ clusters have been calculated and their variation with the increase in x has been analysed. The study should be useful in the design of hybrid BCN-based nanostructures for possible applications analogous to purely carbon-based nanostructures.