Does anyone know of an audio filter in FCP X that removes or at least minimizes echo in a dialogue track?

iZotope RX can do it to a limited degree depending on the source material. But even with that I'd recommend you do that in iZotope's stand-alone app and then import the result into FCPX because FCPX will just apply the filter and try to do everything in real time which can result in some unpleasant-sounding audio (it does not render audio like FCP7 did).

FX Factory (https://fxfactory.com) sells CrumplePop's EchoRemover.

I've found it to be a really nice tool that helps with recordings made in big echo-filled rooms. Obviously it's best to just record a good audio track, but I am not always the one who supervises the shoots I edit. ::sigh::

Sliders to fine tune the sound, and informative tutorials in the FX Factory app's page for EchoRemover.

Best of luck!

Thanks, Joe & Joel.