A novel user-clustering (UC) based downlink hybrid-nonorthogonal multiple access (hybrid-NOMA) assisted heterogeneous network (HetNet) design strategy in the presence of an eavesdropper is explored to investigate the secure resource allocation beyond fifth-generation/sixth-generation (B5G/6G) wireless communications. We have formulated a secrecy rate (SR) maximization problem based on the proposed strategy subject to key performance indicators (KPIs) such as the admission of users in clusters, the association of users with the base station (BS), allocation of power to the users, and user's minimum rate requirement. This novel problem is solved to find an ε-optimal solution within ε = 10⁻³ using the outer approximation algorithm (OAA). This paper performs an analysis of the performance of the proposed strategy and it is depicted through simulation results that the proposed strategy maximizes the SR in terms of KPIs. A performance comparison of the proposed strategy with the existing orthogonal multiple access-only (OMA-only) and nonorthogonal multiple access-only (NOMA-only) strategies is conducted, and it is identified through simulation results that the proposed strategy performs best among the existing strategies. Additionally, a comparison of the performance of the proposed strategy for HetNet and macro base station-only (MBS-only) networks is conducted, and the simulation results depicted that the proposed strategy performs better in HetNet compared to the MBS-only network.