If as you say it goes into a cast iron pipe and you cannot see any fittings on the pipe then I would suggest the pipe is lead. In that case the end cap you suggested will not work as the clip will simply squash the pipe together. If you can get to the front of it squash the pipe together to for a straight seam, scrape as much oxide as possible off the seam, apply flux and heat carefully with blow torch to form a weld on the seam.

Richard