is there a way to re-download a song i have purchased a while ago and now i can't locate it?

I apologize, I posted how to restore in- app purchases

To restore your music:

First check they are not hidden 

Hide and unhide Music ...

https://support.apple.com/en-ca/HT208167

Or ....

Signed in with the Apple ID you made the purchase with

Go to the iTunes Store to find the music, tap the cloud icon to download 

So long as itscstill in the iTunes Store:

To restore purchases on iOS platform (iPad, iPhone and iPod Touch)

1. Tap Settings > iTunes & App Stores
2. Tap your Apple ID and password
3. Tap Sign Out
4. Enter your Apple ID (same used to purchase)
5. Open the app and tap Options > Restore purchases 
6. Confirm your password if necessary 
7. Get back to clips screen and tap icons to download

While signed-in, try to find the tune in the music store -- if it's there, then there should be a cloud icon next to it -- click on that and it should download to your machine.