The transmission coefficient and effective permittivity of frequency selective composites with embedded long conductive fibres have been investigated experimentally and numerically at microwave frequencies. A cluster effect due to the overlapping of fibres is proposed to explain the dispersive microwave properties of composites with randomly distributed fibres. It is observed that the fibre clusters have multiple resonance frequencies that are related to the lengths of the cluster or those of the individual fibres. Interaction between overlapping fibres in a cluster cannot be eliminated by electrical isolation. Cluster effect can explain the appearance of broad resonance peak found in randomly distributed fibres, though the resonance frequency remains close to that of single fibre. This behaviour can be used in the design of composites with broadband frequency selective properties.