Data aggregation is a promising method for conserving energy in Wireless Sensor Networks (WSNs). The most efficient approach to aggregation is organizing the network into clusters. However, this approach which achieves good performance in periodic data collecting applications is proved to be inappropriate for event-driven applications, such as realtime data gathering. In fact, in this case maintaining clusters turned to be very expensive in term of message overhead. To deal with this problem, we propose a clustering scheme suitable for event-driven applications. We show that if the structure is adapted to the occurrence of events, structured approaches can provide good results in real-time WSNs. Simulation results show that our approach performs well both in aggregation gain and energy consumption.