Cell-Free Massive MIMO is believed to be one of the most promising designs of future wireless networks due to its ability to provide more coverage, high reliability and improved spectral efficiency, while also supporting secure communication through distributed access point (AP) cooperation. However, secure downlink transmission in cell-free massive Multiple-Input Multiple-Output (CF-mMIMO) systems becomes challenging when an active eavesdropper (Eve) is located near the targeted user. To achieve high secrecy spectral efficiency (SSE) with a limited number of active APs and a restricted power budget under Nakagami-m fading and imperfect channel state information (CSI), a secrecy-oriented AP selection algorithm is proposed with maximum-ratio transmission (MRT) precoding. Two power allocation schemes are considered: equal power allocation (EPA) and water-filling (WF). Simulation results indicate that the proposed joint-optimization AP activation method enhances the secrecy rate for the targeted user compared with non-optimized selection strategies. For CSI estimation error nk = 0.1, the proposed method achieves SSE improvements of 149% with WF and 317% with EPA. At a higher CSI estimation error nk = 0.4, the improvements are 144.5% with WF and 280.6% with EPA.