Important American fringe event that started everything (3,4)
I believe the answer is:
big bang
'event that started everything' is the definition.
(event that began the universe)
'important american fringe' is the wordplay.
'important' becomes 'big' (similar in meaning).
'american fringe' becomes 'bang' (I've seen this before. I am not sure about the 'american' bit.).
'big'+'bang'='BIG BANG'
Can you help me to learn more?
(Other definitions for big bang that I've seen before include "Our origins in theory" , "Theoretical first ever moment?" , "beginning to matter?" , "Stock Exchange reform" , "major report" .)