<p>Time-Triggered Ethernet (TTEthernet), compliant with SAE AS6802, is crucial for deterministic communication and precise clock synchronization in Industrial Internet of Things (IIoT) systems. However, its deployment in diverse, interconnected IIoT environments exposes critical clock synchronization mechanisms to a significantly broader and more sophisticated spectrum of cyber threats, including insidious latency manipulations, spoofing, and Byzantine failures, which exceed the scope of traditional fault-tolerance models. These threats can severely compromise system determinism, operational integrity, and safety. This paper presents a comprehensive analysis of these security challenges, detailing the evolving threat landscape and evaluating existing defensive strategies. Critically, it explores the potential of emerging technologies such as AI/ML, Distributed Ledger Technology (DLT), Zero Trust Architecture (ZTA), and Post-Quantum Cryptography (PQC) to establish robust, multilayered security. The analysis culminates in the proposal of an innovative, four-layered, adaptive Multilayered Defense Framework designed to safeguard TTEthernet clock synchronization against intelligent and adaptive cyber-attacks. This framework strategically integrates advanced security measures while balancing the inherent need for real-time performance and practical deployment. The findings emphasize that while the inherent fault tolerance of SAE AS6802 is foundational, it is insufficient against intelligent cyber-attacks, necessitating this holistic and adaptive security posture.</p>