This letter investigates secure transmission in non-orthogonal multiple access networks with an untrusted amplify-and-forward relay. In order to enhance secrecy performance, a cooperative jamming strategy is designed to confuse the relay. The new exact and asymptotic expressions for effective secrecy throughput (EST) are derived over Nakagami-m fading channels, which demonstrates that jamming plays an important role for obtaining nonzero EST and should be carefully designed for achieving better secrecy performance. In addition, simulations are presented to provide some insights into parameters, i.e., transmit powers and fading parameters, on the secrecy performance.