Smart cities driven by the Internet of Things (IoT) transform urban environments through data-driven service and intelligent automation. These systems remain subject to cyberattacks, data manipulation, and privacy breaches due to centralized IoT architectures and limited resources. In this paper, a lightweight two-tier blockchain framework is proposed which is designed specifically for secure and scalable smart city environments. The proposed model combines a local immutable ledger for low-overhead and fast processing at the edge, along with cluster-based overlay blockchain which ensures distributed trust and accountability. The key development is the separation of data flow and transaction control which enables parallelism and hereby reducing service latency. The simulations were conducted using Cooja simulator for resource-constrained IoT devices and NS3 for 40 nodes overlay which demonstrates the feasibility of the approach. The performance analysis of the proposed model presents that the model achieves lower packer overhead and end-to-end delay, when compared with existing models. Moreover, acceptable energy and processing overheads are introduced by the cryptographic operations. This paper also presents a threat model and explains how the model addresses confidentiality, integrity and availability concerns. The proposed model achieves 17% increase in processing time in comparison with baseline models but achieves 21% lesser packet overhead in comparison with secure monolithic models. These observations establish the proposed model as a practical and secure solution for next-generation smart city deployments.