NYSC monthly clearance letter simply means you’re active in your Place of Primary Assignment and your employer has given you the go-ahead to receive the monthly allowance. A sample image is available in this post.
This post will show you the history of NCCF as well as the lovely NCCF family song. NYSC means nothing without passing through NCCF. If you know the beginning, you will appreciate NCCF and what she stands for.