Edit echo in fcp x

Two audio channels? Was lav feeding both or maybe shotgun was picking up reverb and you can kill that channel?
Echo/Reverb is generally not easy to remove even in a Digital Audio Workstation (DAW). The problem is the echos are generally the same frequency as the source sound so they a difficult to isolate. There are a number of tricks, not specific to FCPX but have a varying degree of effect.

This is one odd tip I've heard about.

Set up a mike right in front of one of your monitor speakers. Hang a damp dish towel over the speaker and re-record your audio. If the towel is too thick or too wet, you'll get crap, but if you play around you can actually lose quite a bit of echo as the weaker signal will be damped down-- to coin a phrase.

You'll want to get the monitor and mike as far away from the computer as you can get-- or even dump the audio to a cd and record from your stereo system. The fan noise will wreck the whole thing.

If you think about what the damp cloth is doing you can do this to some extent with a filter. Compressing the lower levels maybe. Basically it's trying to crush the weaker signals.