"Yet to be defined" is more grammatically correct, but "still to be defined" is OK and is probably used more in conversation. I'm not sure if "defined" is the word you are looking for though. Do you perhaps mean "confirmed"?

Both are true... but I need to tell you something about yet... we usually use (yet) at the beginning and end of the sentence.... then it' s better to use still in this kind of sentence...

Both are correct. The 'yet' form is more formal. However, I suspect the sentence you are looking for is this: His flight is still to be confirmed.