How to sing more clear?

I don't how to put this in words so I'll use examples. Bruno Mars has a real... Clear voice. I've started singing 2 months ago and I noticed my voice getting clearer, too. But my voice sounds ugly when I sing lower, I want to improve that and make it sound clearer in order to remove that ugly sounding voice. Is this possible? Also, I can sing pretty good without instrumentals, but when I've got instrumentals I sound horrible. How can I fix this?