I have got a few songs for which I have the combined vocal and instrumental and instrumental only. Potentially using the instrumental part, how can I get an acapella track?
Sure. Import both songs (File > Import) and Audacity will place them one above the other and play them both at once.
Select the instrumental by clicking just above the MUTE button. Effect > Invert.
The instrumental is now “opposite” the music in the mix.
Use the Time Shift Tool (two sideways black arrows) and push the instrumental so it perfectly lines up with the mixed song. You may have to change the volume of the instrumental as well to get good cancellation.
This only works well if the instrumentals in the two tracks really are identical.
It can have problems if both songs are highly compressed internet downloads or MP3. MP3 gets its small file sizes by creating sound damage and the damage is likely to be different between the two versions.