Can FCPX remove vocal that's bled into instrument track?

I don't believe FCP has any tools to do this kind of audio processing. Logic Pro can apparently do this. Extract vocal and instrumental stems with Stem Splitter in Logic Pro for Mac - Apple Support There are AI tools that purport to do this like lalal.ai but there are probably others.

If you are running macOS Sonoma 14.4 or later ,and you want to give Logic Pro a try , then there is a 90 day free trial from Apple.

No payment card details required, no cancellation required, it will just expire after 90 days from initial launch.

Logic Pro for Mac - Free Trial - Apple