How can I fix a small leak in PVC pipe?
The only way to properly fix it is to redo that section of pipe.  Anything else is just a band aid that will likely fail in the near future.

The leak is probably worse than you think since drains are not under any pressure.  If you don't fix it, what will likely happen is that as the drain slowly clogs, the leak will get worse as  it has no where to go but out through the leak.