Recently, fast Fourier transform filter-bank multicarrier (FFT-FBMC) modulation was proposed to enable diverse multiple-input multiple-output (MIMO) techniques with FBMC by eliminating the intrinsic interference. A per-subcarrier (PSC) channel equalizer was derived under the assumption of low frequency selectivity. However, this equalization scheme cannot be directly applied to highly frequency selective channels without performance degradation. In this letter, we analyze the received signal after demodulation without any assumption on the channel selectivity. We develop a general closed-form expression for the equivalent channel seen at the demodulator. Furthermore, we design the associate channel equalizer. Simulation results demonstrate that the developed equalizer performs as equally with the previous PSC equalizer in the case of low frequency selectivity, and outperforms the PSC equalizer in highly frequency selective channels.