In this paper, we propose a novel transmission power control scheme to improve energy efficiency in high density clustered wireless sensor networks(WSNs). Our scheme is based on cross-layer strategy that combines both medium access control (MAC) layer and physical (PHY) layer. The algorithm consists of an efficient time slot allocation and opportunistic transmission power control to support energy-efficient intracluster communication. Performance evaluation results reveal that our approach is more energy efficient and can maximize the network lifetime in high density clustered WSNs. Our approach can achieve a network lifetime of up to 2 times longer than that generated by the current schemes. Furthermore, our approach outperforms traditional layered protocols.