Secure data aggregation still represents a very challenging topic in wireless sensor networks' research. In fact, only few solutions exists to face, simultaneously, confidentiality, integrity, adaptive aggregation, and privacy issues. Furthermore, proposals available in literature mainly assume flat network architectures, without leveraging the peculiarities of clustered wireless sensor networks, which are very common in real life deployments. This work tries to bridge the gap by proposing a solution, namely SETA, tailored to a hybrid architecture composed of wireless sensor nodes and wireless mesh routers as cluster heads. In SETA, to lower the processing workload of sensor nodes, only cluster heads are allowed to perform integrity verification checks and message merging operations to face possible network congestions. To prove its effectiveness, it has been compared, using simulations, with respect to DyDAP in several realistic settings. Results have shown that it can provide a slight improvement of the robustness to malicious nodes and of the sensing accuracy, while increasing the overall energy efficiency and decreasing the signaling overhead in the network.