That doesn't sound like a natural native-speaker sentence to me. Are you sure that you've written it down correctly? Is it perhaps 'There's one thing left to say ..' ?

Many singers say things in songs so that they sound different or creative, however, be careful, as this is not correct English.

It should be as Su.Ki says or I have one thing left to say before we go shopping. 

Yes. It's true. 

Really, it is a sentence that I extracted by a song:

"It's just one thing left to say before we fade to love".

Thanks for your comments!

Thanks for everybody!

In my opinion it is metaphor. I would understand it so.. "It is necessary to say something as a final word and close this topic that we can move for another activity"