Yet another reason for me to completely dismiss PFF rankings.
PFF is a great site for confirming what it is you think you see, or don't see. They've improved greatly over the past few seasons as they now have access to coaching video. With that said, no grading system is perfect. The analyst is guessing what a players responsibility is on a particular play and that leaves room for error. They also default in the players favor on a play if they cannot see enough to give him a + grade. It's not perfect, but it is the best we've got.
My take on Clemmons is simple. You either keep him and let him grow into the position, and form a partnership with the other defensive backs because you believe he has not reached his ceiling.
You coach him up, for example;
Taking proper attack angles can be coached. What can't be coached is ball instincts. You either have that aggressiveness, and instinct or you don't. However, with coaching, and experience he might improve to the point he is in better position to make an impact here even without his superior instincts.
I am not convinced he'll improve greatly, but I do think he is underestimated by some. He plays the run extremely well, and is a pretty sure tackler.