Nearly impossible to remove "parts" of mixed-to-stereo song. IF the synth part was mixed down the middle, in mono, it may be possible to minimize it, just like a "vocal remover". These basically flip the phase of one of the channels so anything that is mixed to the center cancels somewhat. That means, usually, kick drum and bass, possibly lead vocals. A signal out of phase with itself tends to cancel.
You can get "Vocal Remover" software, but it basically just does what you can do in nearly any DAW program. Just flip the phase of one channel only. Assuming the sounds in the center are the same amplitude in both channels, it should minimize those sounds. If it was recorded strongly to the center, but has perhaps reverb going right or left, or both, you may be left with minimized main sounds, but fairly noticeable reverb.
If a sound such as a synth is panning from side to side, it may be strong at extreme left and right, and start disappearing as it gets nearer the center.

This is what some cheezoid Karaoke machines do, and why they sound so bad. I'm not an authority, but it may be that "professional" Karaoke discs may either be close covers, or could actually have been taken off the original mix, with vocals removed. I don't know. Either way, it's annoying....to me, anyway.

I don't know this for sure either, but it may be that some "professionally" remixed songs by big-name people may have had access to multitrack versions of the tunes, with the complete permission and associated legal tanglings of the artists, publishers, labels, etc. That way they can keep the fidelity and performances they want to avoid having to create, just mute tracks they don't want, add their own static, unimaginative, overdone, machine-generated, looped drumbeats, blips and noises, and make the record company even more money on yet another version of a song that was on its way to the bargain bin before it was reconstituted as a "dance remix".

OOOPS! :oops: My bias is showing! That sounded kind of cynical, didn't it? That's what happens when a city-fied, middle-aged country boy guitar player gets involved in "sampling" discussions.

But, I digress. People can actually learn a lot about song construction, mixing techniques, and sound design doing just what you want to do. And, if you can make some money at it, no skin off my nose. Go for it! :wink:

And yes, you'll have to consider all the legal ramifications to use someone's music. Google up "sampling", "copyrights", etc. There are even books that discuss this stuff. Maybe even search through ASCAP and BMI to see if they have info. You can WORK on ANYTHING legally, as long as you don't try to distribute it until you have things ironed out. (Actually, maybe not even then, technically, but who will know if it's just on your computer?) No reason to not start to experiment....except you may not be able to afford the usage, and they may not even grant you the usage...so you will have spent a lot of time on something you can't sell. BUT, you will probably have learned something.

The best you may do with what you asked is probably a phasey sounding approximation of the tune. You may even find that if it removes the kick and snare, you can substitute some more techno-sounding beats, and add some other noises. You may be able to chop it up, lengthen it, do some weird effects, pitch-change...whatever. Who knows? Have fun. Just don't post it or try to sell it until you have clearance.

As always, any factual errors, misstatements or omissions are the result of misinformation, subject ignorance, or misfiring neurons on my part. Confident am I that I will be corrected. :?

Good luck,
The kurmudgeonly,
Kapt.Krunch
(Get that digital tone-sucking multi-effects box OUTTA my signal path....and plug me into a REAL amp, son!)