Is "what's left to do is..." grammatically correct? What is a right way to say it?
The best way of saying it is probably "What's left to do is to clean up the mess" or "All that is left is to clean up the mess."
By adding the "to do" the sentence is given a context which is more useful and grammatically stronger.
By taking away the "," in the second sentence, makes the sentence flow better.
What is left is to clean up the mess
What's left is to clean up the mess
What remains to be done is to clean up the mess
are all correct but should be written without the comma.