How can I get rid of smell in tub drain?

You can try putting baking soda and then vingar down the drain. It might take a few tries but I think it will work.

I put baking soda down my drain followed by vinegar. That helps keep the drain clear and makes it smell better at the same time.

If it isn't used very much, the trap is probably drying out and letting sewer gas come up. You need to keep water in the trap of any drains that aren't used regularly. You could try putting some baking soda, then vinegar down the drain, let it sit for a while, then running a bunch of really hot water or boiling water through it to flush it down the pipes to clean any yuck out of the pipes. If the tub isn't used weekly, run a little water down the drain to keep water in the pipe. I had one in a house a long time ago that was for a future bathroom and I had to do that because it never had water running down it.

check the sewer vent (on roof) to see if it's blocked/clogged. Put Dawn dish soap in drain then add baking soda then a little vinegar-fizzing- let sit in drain the longer the better then flush with 1 gallon boiling water.