Major report reveals theory of origin (3,4)
I believe the answer is:
big bang
'major report' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for big bang that I've seen before include "Theoretically, the first thing" , "Theory of world creation" , "Universe's birth?" , "Moment one" , "Stock Exchange reform" .)