Not all events are regular (4)
I believe the answer is:
even
'regular' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for even that I've seen before include "Level, straight or regular" , "neck and neck" , "Unvarying" , "50-50" , "Iron - uniform - flat" .)