A sensor network forms a loosely-coupled distributed environment where collaborative processing among multiple sensor nodes is essentail in order to compensate for the limitation of each sensor node in its processing capability, sensing capability, and energy usage, as well as to improve the degree of fault tolerance. Due to the sheer amount of nodes deployed, collaboration is usually carried out among nodes within the same cluster. Different clustering protocols can affact the performance of network to a great extent. Most existing clustering protocols either do not adequately address the energy-constraint problem or derive clusters proactively which may not be suitable for event-driven collaborative processing in sensor networks. This paper focuses on the design of clustering protocols for collaborative processing. We propose a decentralized reactive clustering (DRC) protocol where the clustering procedure is initial only when events are detected. It uses power control technique to minimize energy usage in forming clusters. We compare the performance of DRC with another popular clustering algorithm, LEACH. Simulation results show considerable improvement over LEACH in energy conservation and network lifetime using DRC.