How can I mute out one word?

[Mike Kujbida]

Is this audio only or a video clip?
If it’s audio only, then split the clip at the offending word and remove it.
If it’s a video clip, it’s a bit trickier.
While you can do it the way you’re thinking of, if there was any kind of room sound, it will be noticeable when the volume drops out.
Grab a short piece of “room tone” without any speaking, copy it and paste it on top of the offending word.
It may still sound strange but, short of re-recording it, you don’t have a lot of choices.