Isolating a particular instrument in an ensemble piece

Do you mean for playback?

If you wish to hear only that instrument during playback, you can use the Mixer to make it play 'solo'.
See:
https://musescore.org/en/handbook/mixer#mute-solo

Or if you mean you want to print the individual part, see File / Parts, and consult the online Handbook for more info on parts (or the Mxier).