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Non-orthogonal multiple access (NOMA) is considered to be one of the promising multiple access (MA) techniques for 5G networks that can provide low latency, high system throughput, and massive connectivity. This paper studies an underlay cognitive radio (CR)-NOMA relaying network, where a secondary source communicates with M NOMA secondary destination users (SDUs) by a means of a detect-and-forward secondary relay under imperfect channel state information conditions. We derive generalized outage probability (OP) expressions in closed form for M NOMA-enabled SDUs. Moreover, we evaluate the OP of the CR-NOMA and conventional CR orthogonal MA schemes to show the superiority of the former. Finally, simulations are performed to corroborate the accuracy of the analytical results.