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Rhonda1216

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Horigan

If it were me, I would buy a stainless steel hose clamp that's the size of the original cracked nut (see pic below).  Put the nut back on the black elbow, put the hose clamp on it and tighten it so it can't expand any more.  Then tighten the nut back onto the white drain and you're done.  The alternative is what a plumber would do;  cut the black straight pipe behind the elbow, buy a new black elbow and a straight section of pipe, put a black sleeve over the two ends of the black pipe and glue it together with PVC cement.  It will be tough to cut the pipe against that wall.

Grumpy_G

Hard to tell from the picture but try to unscrew the ring at the other end of the elbow and remove the elbow. If you're lucky you can slip the ring on that way.

Horigan

I suspect that is glued on, but if it unscrews, that would be preferred.

KK1L

To expand on @Grumpy_G...is the red circle below a glued joint or is it one of the same "contraptions"? If glued, then "what the plummer would do" is the right answer.

To expand on @Horigan it is not clear if the contraption split so that it is not a solid ring anymore, or if it split and became two rings. If the first, then here is a picture of what is suggested, but done on at the base of a faucet.

KK1L

Oh...and if it cracked because you could not get it to stop leaking and were going for just a little tighter, then it could be you are missing or need a new rubber gasket/washer at the joint.