In English you can most definitely call a regime defunct. We sometimes emphasise that the regime no longer exists by saying "the now-defunct regime...". Maybe this is clear for me as a British person. If this BBC link works for you: http://news.bbc.co.uk/today/hi/today/newsid_9582000/9582894.stm The bbc wrote about " senior members of the Libya's defunct regime "

Well, we understand the expression (in the US), but I don't hear it very often. The word "regime" alone is common.

Thanks Carla

No, I've never heard of this expression used, although occasionally it could be used. It's definitely not a commonly used expression, unlike the expression "failed state." It would be helpful to know the full sentence.