Small leak has appeared in a PVC drain pipe joint, how to seal?
It looks like they put the primer down without the glue.   Clean area, reprime it, get pvc glue, put glue around and jam it in.
If the length is not great enough to jam in - it looks a little short or something else moved - then cut about a foot up buy a double female straight joint in that size and if you made your cut straight you can probably use the exact same piece!   The joint will give you a good 1-1.5 inches.   You can always get 16&quot; in that size (4?) if you don't want to go back to the store.
This is not a location that normally would EVER leak, so obviously installed wrong.   Should be a 30 minute fix, if that.