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In this letter, we derive an expression for the secrecy outage probability of a multiuser underlay downlink in which a selected idle secondary user serves as a friendly jammer to enhance physical-layer secrecy of secondary downlink communication in the presence of a passive eavesdropper. Secondary nodes constrain their transmit powers to ensure that the interference caused to the primary network is below an interference temperature limit I _P . We show that careful apportioning of IP between the secondary source and the selected jammer (which transmit simultaneously), and judicious choice of peak powers, is the key to improving secrecy performance. Computer simulation results demonstrate accuracy of the derived expressions.