I'm putting together a showreel for a production company and they want it cut to a popular song from the top 40, having seen every other company out there with a reel cut to pop music. Can someone let me know if its okay to use the music just for promotional purposes. Does it change anything if its a purely instrumental version of the track? am i going to have to resort to Creative commons music or buying something from stock?
No, you cannot use any commercial music without clearing the rights first (which means paying). I have used instrumental versions of some top 40 songs and they were cleared in exactly the same way as the full versions would have been, so I don't believe that makes any substantial difference.
There are times that you can use music without paying under "fair use", but they are quite strictly defined (news, criticism, education, etc) and a company's showreel is definitely not one of those.