I dont use FCP (cant stand it - AVID boy myself) but there will be a hum filetr in the set of audio filters. You will probably need to select the mains freq youre on (50hz 60hz) and it should rip it off fairly well.
Dedicated softwares like Isotope RX and spectral layers are designed for this sort of use and will do a better job, at a cost.
To elaborate on Peter's answer with FCP open in front of me, in the "Hum Remover" filter you have a few controls. Frequency is not preset and can be scanned from 20Hz to 15kHz. You'll still want to use either 50 or 60 to remove mains hum, depending on electrical standards where you recorded.
The "Q" setting determines how wide your removal notches will be and can be left to preset to begin with. "Gain" determines by how much you're reducing the hum so go as low as you can without losing material you actually want to keep. It's preset to maximum reduction.
Then you have selectable boxes to suppress each harmonic. You'll probably have to tick the first one or two but beware that higher harmonics will start to cut into your real audio you probably want to keep and come in handy mostly with really nasty overdriven hum.
Once you've eliminated the hum, you can try to lower the "Q" setting until it starts rearing its head again, go back up a tad and you'll have preserved as much of the original audio as possible while killing the hum.