A mobile ad hoc network is a short-lived cooperative collection of mobile nodes that communicate with each other without the services of a fixed infrastructure. Each host acts as a specialised router to relay information to other nodes. Near-Term Digital Radio (NTDR) networks, which follow the cluster based design principles, are designed specifically for use in ad hoc networks. A major challenge in the design of these networks is their vulnerability to security attacks. In this paper, we describe the security threats and propose security services to counteract these threats in cluster-based NTDR ad hoc networks. We describe secure schemes for a mobile node to initiate, join and leave a cluster. We also discuss the secure end-to-end communication and group key management related issues for NTDR networks.