Small crack around bath tub drain
You can perhaps use an epoxy resin on it or some sort of hardening crack sealer, especially if it is a fiberglass/composite tub material. However, the chances are unfortunately good that the crack will always be there and you just need to replace it or keep patching it every few years.
I wonder if the crack you're refering to is around the drain itself...........The junction between tub and metal drain.........
OR....... do you have a crack in the tub like Lawrence and the SurfaceDr have guessed.......