Extraordinary rear girl jiggles around for all to see (9)
I believe the answer is:
Here is my best explanation:
'extraordinary' is the definition.
'rear girl jiggles around for all to see' is the wordplay.
I cannot really understand how this works, but
'girl' could be 'ula' and 'ula' is located in the answer.
This explanation may well be incorrect...
(Other definitions for irregular that I've seen before include "Partisan", "Rough - guerrilla", "Unpredictable", "Unusual", "volunteer", "Erratic", "As guerrilla", "commonest verbs?", "Out of the ordinary" and "Contrary to a rule or convention".)