You should understand what the role of Harry Fox is and what a Mechanical License is...
Harry Fox is authorized to clear Mechanical Licenses for a lot if music out there... A quick and easy way (though not the cheapest way) to do this...
Mechanical Licenses come into play when you ate doing a "cover version" of a song... A "cover" means you are "re-recording" the song as it exists (no samples, no "sound alikes)... And you are not changing the lyrics or title or anything like that.
There is something called a "compulsory license" with regard to cover versions... This means that once a song has been released on a record, anybody is free to do a cover version on their own record and they do bot need permisionfrom the original copyright holder. All you need to do is pay the mechanical royalty at the statutory rate to the artist (payable through HFA unless you want to negotiate it down)...
Note: this only applies to cover versions used on an "album"... Nit in a movie or commercial...
An "interpolation" or using part of someone elses song incorporated into your own song does not fall into this category. There us ni compulsory license... There is no statutory rate...
...you need to get permission from the copyright holder... They can say "no, we will not let you release this song"... They can charge you whatever amount of money they want.
You have to ask their peussion and see what they say.