Different from conventional key-based cryptography schemes, physical-layer security (PLS) techniques have drawn much attention recently to realise unconditional security from the information theory perspective. As an important performance metric in PLS, the ergodic secrecy rate (ESR) for a multi-input multi-output wireless communication network over a Nakagami fading channel is analysed. The network is consisted of a multi-antenna transmitter (Alice), a multi-antenna legitimate receiver (Bob), and a multi-antenna eavesdropper (Eve). By using the selective transmission (ST) at Alice and the maximum ratio combining (MRC) at Bob and Eve, an exact expression of the ESR is derived. However, due to the infinite summation, it is very hard to evaluate the ESR performance. To reduce computational complexity and obtain more insights, a lower bound of the ESR is then obtained, which is in a closed form. As special cases, the lower bounds of the ESR for the signal-antenna scenario and Rayleigh fading channel are also obtained, respectively. Numerical results show that the derived expressions of the ESR and its lower bound are very accurate to evaluate system performance.