Are students who cleared boards in supplementary examination eligible?

Hey Raksha !
Yes, they are absolutely elligible to appear for NEET UG even if they clear the Boards Exam through Supplementary.
The only criteria which they must fulfill is :

1. Secure passing marks in each and every subjects you pursued in +2 boards.
2. An aggregate marks of 50% in the group subjects of PCB is a must for the UR Category and 40% in cases of reserved categories.

Hope this helps.

Good Luck !