Peter is right. You will lose more than what you want to get rid of.

If the musical group is new they will not have alot of money for an engineer
and may do a very simple mix. Most will place instrumentals split on left and
right tracks with their main vocalist balanced equally on both L & R tracks.

If you take say the Right track and invert it then sum with the Left track you
may be able to kill the vocalist and anything else that was balanced on both
tracks.

I have made rehearsal CDs using this method. I place this processed track
on the Left track and a mono sum of the original on the Right track. The
singer listens using headphones to the mono sum and plays the instrumental
track on their main amp.